Optical head, diffraction device, objective lens, and optical disc apparatus

ABSTRACT

An object of the present invention is to reduce the influence of a stray light incident on a light receiving device by reflection of an unnecessary diffraction light not contributing to write or read on a disc surface at write or read of an optical disc having a large surface reflectivity with respect to an information recording surface. A position in which a diffraction light with an n-order in an advancing path and the n-order in a returning path, reflected on the information recording surface of a first information recording medium, is focused by a condensing lens is different from a position in which a diffraction light different from the n-order diffraction light in at least one of the advancing path and the returning path, reflected on a surface of the first information recording medium, is focused by the condensing lens.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical head which optically writesor reads information to/from plural kinds of information recording mediasuch as optical discs, a diffraction device and an objective lens usedfor the optical head, an optical disc apparatus including the opticalhead, and a computer, an optical disc player, and an optical discrecorder including the optical disc apparatus.

2. Description of the Related Art

In recent years, with a blue-violet semiconductor laser in practicaluse, a Blu-ray Disc (hereinafter, a BD) as a high density and largecapacity optical information recording medium (hereinafter, referred toas an optical disc) having the same size as that of a CD (Compact Disc)and a DVD (Digital Versatile Disc), has been put to practical use. TheBD is an optical disc which has a protective material having a thicknessof about 0.1 mm and performs write or read using a blue-violet laserlight source having a wavelength of about 400 nm and an objective lenshaving a numerical aperture (NA) up to 0.85.

An HD DVD which has a protective material having a thickness of 0.6 mmand a blue-violet laser light source having a wavelength of about 400 nmand an objective lens having a numerical aperture of 0.65 are used hasalso been put to practical use.

There has been proposed an optical head which has compatibility andfocuses a laser beam onto information recording surfaces of such opticaldiscs having different protective material thicknesses using oneobjective lens so as to write or read information.

Each of Japanese Patent Application Laid-Open (JP-A) No. H7-98431 andJapanese Patent Application Laid-Open (JP-A) No. H10-10308 disclose anoptical head having a focusing optical system which can focus a laserbeam onto optical discs having different protective material thicknessesto diffraction limit.

FIG. 33 shows a configuration example of a conventional optical headdisclosed in the Japanese Patent Application Laid-Open (JP-A) No.H7-98431. In FIG. 33, an optical head 130 includes a light source 101for emitting a red laser beam, a beam splitter 103, a collimate lens104, a hologram lens 105, an objective lens 106, a detecting lens 108,and a light receiving device 109. A DVD 70 is an optical disc having aprotective material thickness of 0.6 mm.

The operation of the optical head 130 which writes or reads informationto/from the DVD 70 will be described. The red laser beam emitted fromthe light source 101 passes through the beam splitter 103 and is thenconverted to a substantially parallel beam by the collimate lens 104.The red laser beam passes through the hologram lens 105 and is thenfocused as a light spot onto an information recording surface of the DVD70 beyond a protective material by the objective lens 106. The red laserbeam in a returning path reflected on the information recording surfaceof the DVD 70 passes through the objective lens 106, the hologram lens105, and the collimate lens 104 in the same optical path as an advancingpath. The red laser beam in the returning path is reflected by the beamsplitter 103 and is then given predetermined astigmatism by thedetecting lens 108. Finally, the red laser beam in the returning path isled to the light receiving device 109 so as to generate an informationsignal and a servo signal.

The operation of the optical head 130 for performing write or readto/from a CD 80 as an optical disc having a protective materialthickness of 1.2 mm will be described with reference to FIG. 34. The redlaser beam emitted from the light source 101 passes through the beamsplitter 103 and is then converted to a substantial parallel beam by thecollimate lens 104. The red laser beam is diffracted by the hologramlens 105 and is then focused as a light spot onto an informationrecording surface of the CD 80 beyond a protective material by theobjective lens 106. The red laser beam in a returning path reflected onthe information recording surface of the CD 80 passes through theobjective lens 106, the hologram lens 105, and the collimate lens 104 inthe same optical path as an advancing path. The red laser beam in thereturning path is reflected by the beam splitter 103 and is then givenpredetermined astigmatism by the detecting lens 108. Finally, the redlaser beam in the returning path is led to the light receiving device109 so as to generate an information signal and a servo signal.

A focus error signal for performing write or read to/from the DVD 70 andthe CD 80 can be detected by using an astigmatism method, etc. ofdetecting a focal spot given the astigmatism by the detecting lens 108in a four-part split light receiving pattern in the light receivingdevice 109. A tracking error signal can be detected by using what iscalled a three-beam method, a differential push-pull method (DPPmethod), and the like which use a main beam and a sub-beam generated bya diffraction grating (not shown).

The functions of the hologram lens 105 and the objective lens 106 willbe described in detail with reference to FIGS. 35 and 36.

The hologram lens 105 has a grating pattern 105 a as shown in FIG. 35for focusing a very small light spot onto each of the DVD 70 and the CD80. The diffraction efficiency of a plus-first-order diffraction lightof the hologram lens 105 is less than 100%. The hologram lens 105 isdesigned in such a manner that a transmission light (hereinafter, in thepresent invention, a non-diffracted transmission light can also berepresented as a zero-order diffraction light and the transmission lightis handled as one of diffraction lights) has sufficient intensity. Thehologram lens 105 is blazed so as to increase the sum of the amount ofthe zero-order diffraction light and a plus-first-order diffractionlight. The light utilization efficiency can be high.

The objective lens 106 has an NA of 0.6 and is designed so as to form afocal spot onto the DVD 70 having a protective material with a thicknessof 0.6 mm to diffraction limit, as shown in FIG. 36A, when the laserbeam passed through the hologram lens 105 without being diffracted (thatis, the zero-order diffraction light) is incident.

As shown in FIG. 36B, the plus-first-order diffraction light diffractedby the hologram lens 105 is focused onto the CD 80 by the objective lens106. Here, the plus-first-order diffraction light isaberration-corrected so as to form a focal spot onto the CD 80 having aprotective material thickness of 1.2 mm to diffraction limit.

The hologram lens 105 for diffracting part of the incident light iscombined with the objective lens 106 so as to realize a bifocal lenswhich can form a focal spot focused onto each of the optical discshaving different protective material thicknesses to diffraction limit.

Since the hologram lens 105 has lens action, the positions of two focalpoints in the optical axis direction are different. Thus, when a lightspot formed on one of the focal points is used to write or readinformation, a light spot focused on the other focal point is enlarged,which does not affect write or read of information.

With such optical head 130, information can be written or read to/fromdifferent kinds of optical discs using one objective lens.

SUMMARY OF THE INVENTION

Each of the Japanese Patent Application Laid-Open (JP-A) No. H7-98431and Japanese Patent Application Laid-Open (JP-A) No. H10-10308 disclosethe configuration of the optical head which can be compatible withplural kinds of optical discs, such as the DVD and CD, having differentprotective material thicknesses, with the bifocal lens using thehologram lens.

However, the conventional arts in those documents do not describe theinfluence of a stray light that an unnecessary diffraction light notcontributing to write or read is reflected on the disc surface and thenis incident on the light receiving device at write or read of theoptical disc whose surface reflectivity with respect to the informationrecording surface is relatively large, e.g., having a plurality ofinformation recording surfaces.

In the present invention, by defining a lens power given to the hologramlens, an object of the present invention is to reduce interferencebetween a signal obtained by a light receiving device by reflection of apredetermined-order diffraction light used for write or read on apredetermined information recording surface and a signal obtained by thelight receiving device by reflection of a diffraction light of an ordernot contributing to write or read on a disc surface, thereby performingstable information signal detection and servo signal detection.

An optical head according to a first aspect of the present inventionincludes: a light source; a diffraction device configured to diffract alaser beam emitted from the light source so as to generate diffractionlights of plural orders; an objective lens, with n and m as integers,configured to focus an n-order diffraction light generated by thediffraction device onto an information recording surface of a firstinformation recording medium, and focus an m-order diffraction lightonto an information recording surface of a second information recordingmedium; a condensing lens configured to condense the laser beamreflected on the information recording surface of the first informationrecording medium or the information recording surface of the secondinformation recording medium; and a light receiving unit configured toreceive the laser beam condensed by the condensing lens so as togenerate a focus error signal,

wherein the optical path of the laser beam before being reflected on theinformation recording medium is an advancing path, the optical path ofthe laser beam after being reflected on the information recording mediumis a returning path,

a position in the light receiving unit in which a diffraction light withthe n-order diffraction light in the advancing path and the n-orderdiffraction light in the returning path, reflected on the informationrecording surface of the first information recording medium, is focusedby the condensing lens being different from a position in the lightreceiving unit in which a diffraction light different from the n-orderdiffraction light in at least one of the advancing path and thereturning path, reflected on a surface of the first informationrecording medium, is focused by the condensing lens.

An optical head according to a second aspect of the present inventionincludes: a light source; a diffraction device for diffracting a laserbeam emitted from the light source so as to generate diffraction lightsof plural orders; an objective lens, with n and m as different integers,for focusing an n-order diffraction light generated by the diffractiondevice onto an information recording surface of a first informationrecording medium, focusing an m-order diffraction light onto aninformation recording surface of a second information recording medium,and focusing the laser beam reflected on the information recordingsurface of the first information recording medium or the informationrecording surface of the second information recording medium; and alight receiving unit for receiving the laser beam focused by theobjective lens so as to generate a focus error signal, wherein anoptical path of the laser beam before being reflected on the informationrecording medium is an advancing path, an optical path of the laser beamafter being reflected on the information recording medium is a returningpath, and a position in the light receiving unit in which a diffractionlight having the n-order diffraction light in the advancing path and then-order diffraction light in the returning path, reflected on theinformation recording surface of the first information recording medium,is focused by the condensing lens is different from a position in thelight receiving unit in which a diffraction light different from then-order diffraction light in at least one of the advancing path and thereturning path, reflected on the surface of the first informationrecording medium, is focused by the condensing lens.

The optical head according to the first aspect of the present inventionmay be designed so that the n and m are different integers, and theobjective lens focuses the laser beams having the same wavelengthemitted from the light source onto the information recording surface ofthe first information recording medium and the information recordingsurface of the second information recording medium.

The optical head according to the first aspect of the present inventionmay be designed so that the n and m are the same integers, and theobjective lens focuses the laser beams having different wavelengthsemitted from the light source onto the information recording surface ofthe first information recording medium and the information recordingsurface of the second information recording medium.

The optical head according to the first aspect of the present inventionmay be designed so that the n and m are different integers, and theobjective lens focuses the laser beams having different wavelengthsemitted from the light source onto the information recording surface ofthe first information recording medium and the information recordingsurface of the second information recording medium.

The optical head according to the first aspect and the second aspect ofthe present invention may be designed so that a position in the lightreceiving unit in which a diffraction light having an m-orderdiffraction light in the advancing path and the m-order diffractionlight in the returning path, reflected on the information recordingsurface of the second information recording medium, is focused by thecondensing lens is different from a position in the light receiving unitin which a diffraction light different from the m-order diffractionlight in at least one of the advancing path and the returning path,reflected on a surface of the second information recording medium, isfocused by the condensing lens.

The optical head according to the first aspect and the second aspect ofthe present invention may be designed so that a protective materialthickness of the information recording surface of the first informationrecording medium is t1, a protective material thickness of theinformation recording surface of the second information recording mediumis t2, so that t1<t2, and the objective lens focuses a zero-orderdiffraction light onto the information recording surface of the firstinformation recording medium and focuses a plus-first-order diffractionlight onto the information recording surface of the second informationrecording medium.

The optical head according to the first aspect of the present inventionmay be designed so that a position in the light receiving unit in whicha diffraction light with a zero-order diffraction light in the advancingpath and a zero-order diffraction light in the returning path, reflectedon the information recording surface of the first information recordingmedium, is focused by the condensing lens is different from a positionin the light receiving unit in which the zero-order diffraction light inone of the advancing path and the returning path and a minus-first-orderdiffraction light in the other path, reflected on the surface of thefirst information recording medium, is focused by the condensing lens.

The optical head according to the first aspect and the second aspect ofthe present invention may be designed so that a protective materialthickness of the information recording surface of the first informationrecording medium is t1, a protective material thickness of theinformation recording surface of the second information recording mediumis t2, a refractivity of the protective material is n, a firstprotective material thickness of the information recording surface ofthe first information recording medium is t1_L1, a second protectivematerial thickness of the information recording surface of the firstinformation recording medium is t1_L2, a working distance for write orread of the first information recording medium is WD1, a workingdistance for write or read of the second information recording medium isWD2, and a focal position interval difference between a plus-first-orderdiffraction light and the minus-first-order diffraction light is k, thenWD1−WD2≠(t2−t1−2×t1_L1)/n−k, and WD1−WD2≠(t2−t1−2×t1_L2)/n−k aresatisfied.

The optical head according to the first aspect and the second aspect ofthe present invention may be designed so thatWD1−WD2≠(t2−t1−2×t1_L1)/n−k is satisfied.

The optical head according to the first aspect and the second aspect ofthe present invention may be designed so that a position in the lightreceiving unit in which a diffraction light with a plus-first-orderdiffraction light in the advancing path and a plus-first-orderdiffraction light in the returning path, reflected on the informationrecording surface of the second information recording medium, is focusedby the condensing lens is different from a position in the lightreceiving unit in which a minus-first-order diffraction light in theadvancing path and a minus-first-order diffraction light in thereturning path, reflected on a surface of the second informationrecording medium, is focused by the condensing lens.

The optical head according to the first aspect and the second aspect ofthe present invention may be designed so that a protective materialthickness of the information recording surface of the first informationrecording medium is t1, a protective material thickness of theinformation recording surface of the second information recording mediumis t2, a refractivity of the protective material is n, a protectivematerial thickness of the information recording surface of the secondinformation recording medium is t2_ra, a working distance for write orread of the first information recording medium is WD1, a workingdistance for write or read of the second information recording medium isWD2, and a focal position interval difference between theplus-first-order diffraction light and the minus-first-order diffractionlight is k, then WD1−WD2 ≠(2×t2−2×t1−t2_ra)/(2×n)−k is satisfied.

The optical head according to the first aspect and the second aspect ofthe present invention may be designed so thatWD1−WD2>(2×t2−2×t1−t2_ra)/(2×n)−k is satisfied.

The optical head according to the first aspect and the second aspect ofthe present invention may be designed so that a protective materialthickness of the information recording surface of the first informationrecording medium is t1, a protective material thickness of theinformation recording surface of the second information recording mediumis t2, so that t1<t2, and the objective lens focuses a plus-first-orderdiffraction light onto the information recording surface of the firstinformation recording medium and focuses a zero-order diffraction lightonto the information recording surface of the second informationrecording medium.

In the optical head according to the first aspect of the presentinvention may be designed so that a position in the light receiving unitin which a diffraction light having the plus-first-order diffractionlight in the advancing path and the plus-first-order diffraction lightin the returning path, reflected on the information recording surface ofthe first information recording medium, is focused by the condensinglens is different from a position in the light receiving unit in which adiffraction light having the plus-first-order diffraction light in oneof the advancing path and the returning path and a plus-second-orderdiffraction light in the other path, reflected on the surface of thefirst information recording medium, is focused by the condensing lens.

The optical head according to the first aspect and the second aspect ofthe present invention may be designed so that the objective lens is anobjective lens having a diffraction structure integrated with thediffraction device.

The optical head according to the first aspect and the second aspect ofthe present invention may be designed so that an effective region of theobjective lens has a first region having the diffraction structure and asecond region not having the diffraction structure.

In the optical head according to the first aspect and the second aspectof the present invention may be designed so that the first region is aninner periphery region including the optical axis of the objective lensand the second region is a region around the first region.

The optical head according to the first aspect and the second aspect ofthe present invention may be designed so that an effective region of theobjective lens has a first diffraction region in which a predetermineddiffraction structure is formed in the inner periphery region includingthe optical axis of the objective lens and a second diffraction regionin which a diffraction structure different from that of the firstdiffraction region is formed around the first region.

In the optical head in the first aspect and the second aspect of thepresent invention may be designed so that the second diffraction regionis formed in such a manner that the diffraction efficiency of theplus-first-order diffraction light is maximum.

In the optical head according to the first aspect and the second aspectof the present invention may be designed so that the diffraction deviceand the objective lens are separated from each other.

In the optical head according to the first aspect and the second aspectof the present invention may be designed so that a reflectivity of theinformation recording surface of the first information recording mediumis R1 r, and a surface reflectivity thereof is R1 s, then, 0.5≦R1 r/R1s≦3.0 is satisfied.

In the optical head according to the first aspect and the second aspectof the present invention may be designed so that the first informationrecording medium has a plurality of information recording surfaces.

An objective lens according to a third aspect of the present inventionhas a diffraction structure for diffracting a laser beam emitted from alight source so as to generate diffraction lights of a plurality oforders, with n and m as integers, focusing an n-order diffraction lightgenerated by the diffraction structure onto an information recordingsurface of a first information recording medium, and focusing an m-orderdiffraction light onto an information recording surface of a secondinformation recording medium, wherein an optical path of the laser beambefore being reflected on the information recording medium is anadvancing path, the optical path of the laser beam after being reflectedon the information recording medium is a returning path, and a positionin which a diffraction light with the n-order diffraction light in theadvancing path and the n-order diffraction light in the returning path,reflected on the information recording surface of the first informationrecording medium, is focused by a condensing lens for focusing the laserbeam reflected on the information recording surface of the firstinformation recording medium or the information recording surface of thesecond information recording medium is different from the position inwhich a diffraction light different from the n-order diffraction lightin at least one of the advancing path and the returning path, reflectedon the surface of the first information recording medium, is focused bythe condensing lens.

A diffraction device according to a fourth aspect of the presentinvention has a diffraction structure configured to diffract a laserbeam emitted from a light source so as to generate diffraction lights ofplural orders, in which with n and m as integers, an n-order diffractionlight generated by the diffraction structure being focused onto aninformation recording surface of a first information recording mediumand an m-order diffraction light being focused onto an informationrecording surface of a second information recording medium by anobjective lens, wherein an optical path of the laser beam before beingreflected on the information recording medium is an advancing path, anoptical path of the laser beam after being reflected on the informationrecording medium is a returning path, a position in which a diffractionlight with the n-order diffraction light in the advancing path and then-order diffraction light in the returning path, reflected on theinformation recording surface of the first information recording medium,is focused by a condensing lens for focusing the laser beam reflected onthe information recording surface of the first information recordingmedium or the information recording surface of the second informationrecording medium being different from a position in which a diffractionlight different from the n-order diffraction light in at least one ofthe advancing path and the returning path, reflected on a surface of thefirst information recording medium, is focused by the condensing lens.

An optical disc apparatus according to a fifth aspect of the presentinvention includes an optical head, a motor for rotating and driving aninformation recording medium, and a control unit for controlling theoptical head and the motor, the optical head being an optical headdefined in the first aspect and the second aspect.

An optical disc apparatus according to a sixth aspect of the presentinvention include: an optical head having a light source, an objectivelens for focusing a laser beam emitted from the light source onto aninformation recording surface of an information recording medium, alight receiving unit for receiving the laser beam reflected on theinformation recording surface so as to generate a focus error signal,and an actuator for moving the objective lens in a focus direction; anda control unit for controlling the optical head, wherein the controlunit determines the amount of movement of the objective lens based on afocus sensitivity in the relation between an electric current or avoltage to be supplied to the actuator and the amount of movement of theobjective lens and starts focus control when the focus error signalobtained with movement of the objective lens in the direction close tothe information recording medium exceeds a threshold value.

The optical disc apparatus according to the sixth aspect of the presentinvention may further include a memory for storing the focussensitivity, wherein the control unit reads the focus sensitivity fromthe memory.

In the optical disc apparatus in the sixth aspect of the presentinvention, the optical head may further include a memory for storing thefocus sensitivity, and the control unit may read the focus sensitivityfrom the memory.

In the optical disc apparatus in the sixth aspect of the presentinvention, the control unit may measure the focus sensitivity when theoptical disc apparatus is turned on or the optical disc is inserted.

The optical disc apparatus according to the sixth aspect of the presentinvention may be designed so that the control unit measures the focussensitivity using the focus error signal.

In the optical disc apparatus in the sixth aspect of the presentinvention may be designed so that the focus error signal used may be thefocus error signal appearing before the focus error signal used for thefocus control.

In the optical disc apparatus in the sixth aspect of the presentinvention, the control unit may stop the optical disc so as to measurethe focus sensitivity.

In the optical disc apparatus in the sixth aspect of the presentinvention, the control unit may move the objective lens in the directionclose to the optical disc at high speed at first and then, at a speedlower than the high speed.

In the optical disc apparatus in the sixth aspect of the presentinvention, the optical head may further include a spherical aberrationcorrection mechanism and may match spherical aberration with theprotective material thickness of the optical disc before the focuscontrol is started.

A computer in a seventh aspect of the present invention includes: anoptical disc apparatus defined in the sixth aspect; an input means forinputting information; a computation means for performing computationbased on information read from the optical disc apparatus and/orinformation inputted from the input means; and an output means foroutputting the information read from the optical disc apparatus and/orthe information inputted from the input means and/or the result computedby the computation means.

An optical disc player in an eighth aspect of the present inventionincludes an optical disc apparatus defined in the sixth aspect; and adecoder for converting an information signal obtained from the opticaldisc apparatus to image information.

An optical disc recorder in a ninth aspect of the present inventionincludes an optical disc apparatus defined in the sixth aspect; and anencoder for converting image information to an information signal to berecorded by the optical disc apparatus.

The optical head of the present invention can satisfactorily performwrite or read of plural different optical discs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of an optical head for write or readof a first optical disc according to Embodiment 1 of the presentinvention;

FIG. 2 is a schematic block diagram of the optical head for write orread of a second optical disc according to Embodiment 1 of the presentinvention;

FIG. 3 is a diagram schematically showing the configuration of ahologram lens according to Embodiment 1 of the present invention;

FIGS. 4A and 4B are diagrams showing the functions of the hologram lensand an objective lens according to Embodiment 1 of the presentinvention;

FIG. 5 is a diagram showing the relation between the diffractionefficiencies of the hologram lens and a hologram integrated typeobjective lens according to the present invention, the amount of signallights, and the amount of stray lights;

FIG. 6 is a diagram schematically showing the relation between aplus-first-order diffraction light, a zero-order diffraction light, anda minus-first-order diffraction light at write or read of the firstoptical disc according to Embodiment 1 of the present invention;

FIG. 7 is a diagram schematically showing focus error signals at writeor read of the first optical disc or so-called S-shaped signalsaccording to the present invention;

FIG. 8 is a diagram schematically showing the relation between aplus-first-order diffraction light, a zero-order diffraction light, anda minus-first-order diffraction light at write or read of the secondoptical disc according to Embodiment 1 of the present invention;

FIG. 9 is a diagram schematically showing focus error signals at writeor read of the second optical disc or so-called S-shaped signalsaccording to the present invention;

FIG. 10 is a schematic block diagram of an optical head for write orread of the first optical disc according to Embodiment 2 of the presentinvention;

FIG. 11 is a schematic block diagram of the optical head for write orread of the second optical disc according to Embodiment 2 of the presentinvention;

FIG. 12 is a diagram schematically showing the configuration of thehologram integrated type objective lens according to Embodiment 2 of thepresent invention;

FIGS. 13A and 13B are diagrams showing the function of the hologramintegrated type objective lens according to Embodiment 2 of the presentinvention;

FIG. 14 is a schematic block diagram of an optical disc apparatusaccording to Embodiment 4 of the present invention;

FIG. 15 is a schematic block diagram of a computer according toEmbodiment 5 of the present invention;

FIG. 16 is a schematic block diagram of an optical disc player accordingto Embodiment 6 of the present invention;

FIG. 17 is a schematic block diagram of an optical disc recorderaccording to Embodiment 7 of the present invention;

FIG. 18 is a perspective view showing an example of the schematicarrangement state of the component portions of the optical head shown inFIG. 1;

FIG. 19 is an enlarged view showing the vicinity of the focal positionsof the plus-first-order diffraction light, the zero-order diffractionlight, and the minus-first-order diffraction light of the first opticaldisc shown in FIG. 6;

FIG. 20 is a diagram of assistance in explaining the difference betweenthe focal position of the zero-order diffraction light of the firstoptical disc shown in FIG. 6 and the focal position of theplus-first-order diffraction light of the second optical disc shown inFIG. 8;

FIG. 21 is an enlarged view showing the vicinity of the focal positionsof the plus-first-order diffraction light, the zero-order diffractionlight, and the minus-first-order diffraction light of the second opticaldisc shown in FIG. 8;

FIG. 22 is a schematic diagram of the focal positions of theplus-first-order diffraction light, the zero-order diffraction light,and the minus-first-order diffraction light shown in FIG. 6;

FIG. 23 is a diagram of assistance in explaining the state that thefocal position of a higher order diffraction light is matched with thesurface of the second optical disc shown in FIG. 8;

FIG. 24 is a schematic block diagram of an optical head according toEmbodiment 3 of the present invention having the objective lens fordiffracting laser beams having different wavelengths and focusing thelaser beams onto information recording surfaces of different kinds ofoptical discs, in the case that the optical disc is a BD;

FIG. 25 is a schematic block diagram of the optical head according toEmbodiment 3 of the present invention having the objective lens fordiffracting laser beams having different wavelengths and focusing thelaser beams onto information recording surfaces of different kinds ofoptical discs, in the case that the optical disc is a DVD;

FIG. 26 is a schematic block diagram of the optical head according toEmbodiment 3 of the present invention having the objective lens fordiffracting laser beams having different wavelengths and focusing thelaser beams onto information recording surfaces of different kinds ofoptical discs, in the case that the optical disc is a CD;

FIG. 27 is a diagram of assistance in explaining the optical head shownin FIG. 26 in which the focal position of the plus-first-orderdiffraction light is close to the surface of the optical disc;

FIG. 28 is a flowchart showing a focus control operation executed by acontrol unit in the optical disc apparatus shown in FIG. 14;

FIG. 29 is a diagram showing that when the focus control operation shownin FIG. 28 is executed to the first optical disc, the zero-orderdiffraction light is focused onto the information recording surface ofthe optical disc;

FIG. 30 is a diagram of assistance in explaining the case that the focuscontrol operation shown in FIG. 28 is executed to the focus errorsignals appearing from the first optical disc;

FIG. 31 is a diagram showing that when the focus control operation shownin FIG. 28 is executed to the second optical disc, the plus-first-orderdiffraction light is focused onto the information recording surface ofthe optical disc;

FIG. 32 is a diagram of assistance in explaining the case that the focuscontrol operation shown in FIG. 28 is executed to the focus errorsignals appearing from the second optical disc;

FIG. 33 is a schematic block diagram of a conventional optical head forwrite or read of the DVD;

FIG. 34 is a schematic block diagram of the conventional optical headfor write or read of the CD;

FIG. 35 is a diagram schematically showing the configuration of ahologram lens of the conventional optical head; and

FIGS. 36A and 36B are diagrams showing the functions of the hologramlens and an objective lens of the conventional optical head.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of an optical head, a diffraction device, an objective lens,an optical disc apparatus, a computer, an optical disc player, and anoptical disc recorder according to the present invention will bedescribed below with reference to the drawings.

Embodiment 1

FIGS. 1 and 2 are schematic block diagrams of the optical head accordingto an embodiment of the present invention.

In FIGS. 1 and 2, an optical head 30 includes a light source 1 emittinga blue-violet laser beam, a beam splitter 3, a collimate lens 4, ahologram lens 5, an objective lens 6, a detecting lens 8, and a lightreceiving device 9 for receiving the laser beam. The reference numeral50 denotes a first optical disc as an optical disc having a protectivematerial with a thickness of about 0.075 to 0.1 mm. The referencenumeral 60 denotes a second optical disc as an optical disc having aprotective material with a thickness of about 0.6 mm.

The operation of the optical head 30 which writes or reads informationto/from the first optical disc 50 will be described using FIG. 1. Theblue-violet laser beam emitted from the light source 1 passes throughthe beam splitter 3 and is then converted to a substantially parallelbeam by the collimate lens 4. The blue-violet laser beam passes throughthe hologram lens 5 and is then focused as a light spot onto aninformation recording surface of the first optical disc 50 beyond aprotective material by the objective lens 6. The blue-violet laser beamin a returning path reflected on the information recording surface ofthe first optical disc 50 passes through the objective lens 6, thehologram lens 5, and the collimate lens 4 in the same optical path as anadvancing path. The blue-violet laser beam in the returning path isreflected by the beam splitter 3 and is then given predeterminedastigmatism by the detecting lens 8. The blue-violet laser beam in thereturning path is led to the light receiving device 9 so as to generatean information signal and a servo signal.

The operation of the optical head 30 which writes or reads informationto/from the second optical disc 60 will be described with reference toFIG. 2. The blue-violet laser beam emitted from the light source 1passes through the beam splitter 3 and is then converted to asubstantially parallel beam by the collimate lens 4. The blue-violetlaser beam is diffracted by the hologram lens 5 and is then focused as alight spot onto an information recording surface of the second opticaldisc 60 beyond a protective material by the objective lens 6. Theblue-violet laser beam in a returning path reflected on the informationrecording surface of the second optical disc 60 passes through theobjective lens 6, the hologram lens 5, and the collimate lens 4 in thesame optical path as an advancing path. The blue-violet laser beam inthe returning path is reflected by the beam splitter 3 and is then givenpredetermined astigmatism by the detecting lens 8. The blue-violet laserbeam in the returning path is led to the light receiving device 9 so asto generate an information signal and a servo signal.

A focus error signal for write or read of the first optical disc 50 andthe second optical disc 60 can be detected by using an astigmatismmethod of detecting a focal spot given the astigmatism by the detectinglens 8 on a four-part split light receiving pattern of the lightreceiving device 9. A tracking error signal can be detected by usingso-called a three-beam method and a differential push-pull method (DPPmethod) using a main beam and a sub-beam generated by a diffractiongrating (not shown).

The functions of the hologram lens 5 and the objective lens 6 will bedescribed in detail using FIGS. 3 and 4. In the optical head 30 shown inFIGS. 1 and 2, the hologram lens 5 and the objective lens 6 are unitedas shown in FIG. 18 and can be integrally driven by an actuator 31 in afocus direction 31 a and a tracking direction 31 b without changing theposition relation between them. The actuator 31 is operation-controlledby a control unit 402 to which the servo signal generated by the lightreceiving device 9 is fed. The control unit 402 is provided in theoptical disc apparatus having the optical head 30 explained aslater-described Embodiment 4.

The hologram lens 5 is formed with a concentric grating pattern 5 a. Itscenter, that is, the optical axis is matched with the objective lens 6within an assembly error. The grating pattern of the hologram lens 5 isformed only in the portion having a diameter smaller than an aperturedetermined by the objective lens 6. No diffraction thus occurs in theportion not formed with the grating pattern of the hologram lens 5 atall.

The phase of a zero-order diffraction light (transmission light) in thegrating pattern 5 a has an average value of the amount of phasemodulation given by the grating pattern 5 a. It is desired that this besubstantially matched with the phase of the transmission light in aregion 5 b without the grating pattern, thereby improving the focusingperformance. For instance, when the grating pattern of the hologram lens5 is of relief type, as shown in FIG. 3, it is desired that the heightof the surface of the region 5 b without the grating pattern be matchedwith the substantially average level of the concaves and convexesforming the grating pattern 5 a.

The diffraction efficiency of a plus-first-order diffraction light ofthe hologram lens 5 is less than 100%. The hologram lens 5 is designedin such a manner that the transmission light (zero-order diffractionlight) has sufficient intensity. The hologram lens 5 is blazed so as toincrease the sum of the amount of the zero-order diffraction light andthe plus-first-order diffraction light. Therefore the light utilizationefficiency can be high.

When the hologram lens 5 is used in the optical head only for read ofboth the first optical disc 50 and the second optical disc 60, it isdesired that the diffraction efficiency of the plus-first-orderdiffraction light be about 30 to 70%. The diffraction efficiency is setin this manner, so that information can be read from the first opticaldisc 50 and the second optical disc 60 by using a substantially equalamount of light. Thus the output of the light source can be reduced.

When the hologram lens 5 is used in the optical head which performs onlyread of the second optical disc 60 and can perform write and read of thefirst optical disc 50, it is desired that the diffraction efficiency ofthe plus-first-order diffraction light be 30% or below. The diffractionefficiency is set in this manner, so that the transmittance (thediffraction efficiency of the zero-order diffraction light) of thehologram lens 5 can be increased. Accordingly, the light utilizationefficiency with respect to the first optical disc 50 for write can behigh. The output of the light source at write can be reduced.

The objective lens 6 has an NA of 0.85 and is designed so as to form afocal spot onto the first optical disc 50 having the protective materialthickness of about 0.1 mm to diffraction limit, as shown in FIG. 4A,when the laser beam passed through the hologram lens 5 without beingdiffracted (that is, the zero-order diffraction light) is incident.

As shown in FIG. 4B, the plus-first-order diffraction light diffractedby the hologram lens 5 is focused onto the second optical disc 60 by theobjective lens 6. Here, the plus-first-order diffraction light isaberration-corrected so as to form a focal spot onto the second opticaldisc 60 having the protective material thickness of about 0.6 mm todiffraction limit.

The hologram lens 5 for diffracting part of the incident light iscombined with the objective lens 6 so as to realize a bifocal lens whichcan form a focal spot focused onto each of the optical discs havingdifferent protective material thicknesses to diffraction limit.

The hologram lens 5 of this embodiment has the effect of adding theconcave lens power to the plus-first-order diffraction light withrespect to the zero-order diffraction light. Thus the focal position ofthe plus-first-order diffraction light passing through the objectivelens 6 is farther than that of the zero-order diffraction light. Whenthe plus-first-order diffraction light is focused onto the informationrecording surface of the second optical disc 60 having the protectivematerial thickness larger than that of the first optical disc 50, aworking distance (WD) as the interval between the second optical disc 60and the objective lens 6 can be sufficiently secured.

As described above, the hologram lens 5 provides the concave lens actionto the plus-first-order diffraction light. Thus, the positions in theoptical axis direction of two focal points of the zero-order diffractionlight for write or read of the first optical disc 50 and theplus-first-order diffraction light for write or read of the secondoptical disc 60 are different. Accordingly, when information is writtenor read using a light spot formed on one of the focal points, a lightspot focused on the other focal point is enlarged, which does not affectwrite or read of information.

When the grating pattern is formed so as to generate the desiredplus-first-order diffraction light by the hologram lens 5, in additionto the zero-order diffraction light and the plus-first-order diffractionlight, a minus-first-order diffraction light conjugating with theplus-first-order diffraction light and a higher order diffraction lightare generated. For example, the hologram lens 5 of this embodiment, whenthe diffraction efficiency of the zero-order diffraction light(transmission efficiency) is about 65% and the diffraction efficiency ofthe plus-first-order diffraction light is about 15%, the diffractionefficiency of the minus-first-order diffraction light is about 10%.

As shown in FIG. 5, a thing used for read and write of the first opticaldisc is a light spot (in the drawing, a “signal light B”) which isformed by the laser beam passing through the hologram lens 5 in theadvancing path (zero-order diffraction light in the advancing path),reflected on the information recording surface, then passing through thehologram lens 5 again in the returning path (zero-order diffractionlight in the returning path), and finally detected by the lightreceiving device.

On the other hand, a thing used for read and write of the second opticaldisc is a light spot (in the drawing, a “signal light H”) which isformed by the laser beam passing through the hologram lens 5 in theadvancing path (plus-first-order diffraction light in the advancingpath), reflected on the information recording surface, then passingthrough the hologram lens 5 again in the returning path(plus-first-order diffraction light in the returning path), and finallydetected by the light receiving device.

Other light spots detected by the light receiving device (that is, straylights) only in a combination of the zero-order diffraction light, theplus-first-order diffraction light, and the minus-first-orderdiffraction light are:

stray light (1) (advancing path: zero-order diffraction light bytransmission→returning path: plus-first-order diffraction light bydiffraction);

stray light (2) (advancing path: plus-first-order diffraction light bydiffraction→returning path: zero-order diffraction light bytransmission);

stray light (3) (advancing path: minus-first-order diffraction light bydiffraction→returning path: plus-first-order diffraction light bydiffraction);

stray light (4) (advancing path: plus-first-order diffraction light bydiffraction→returning path: minus-first-order diffraction light bydiffraction);

stray light (5) (advancing path: minus-first-order diffraction light bydiffraction→returning path: zero-order diffraction light bytransmission);

stray light (6) (advancing path: zero-order diffraction light bytransmission→returning path: minus-first-order diffraction light bydiffraction); and stray light (7) (advancing path: minus-first-orderdiffraction light by diffraction→returning path: minus-first-orderdiffraction light by diffraction).

Thus, seven patterns of the stray lights exist.

In FIG. 5, the number described under the signal light or the straylight indicate the total efficiency in the advancing path and thereturning path. In the signal light B used for write or read of thefirst optical disc, the amount of light contributing to actual write orread includes the amount of light passing through the region without thegrating pattern. However, the efficiency shown in FIG. 5 does notinclude the amount of light passing through the region without thegrating pattern.

The unnecessary diffraction lights not contributing to write or read ofthe first optical disc and the second optical disc (the stray lights (1)to (7)) are greatly defocused onto the information recording surface andbecome a very large light spot on the light receiving device, which isnot substantially a problem when the write or read is performed to thefirst optical disc and the second optical disc.

As shown in FIG. 6, the minus-first-order diffraction light conjugatingwith the plus-first-order diffraction light having the concave lenspower has the convex lens power. So, the focal position of theminus-first-order diffraction light is closer to the objective lens 6than that of the zero-order diffraction light.

When the focal position of the minus-first-order diffraction lighthaving the convex lens power formed by the hologram lens 5 is matchedwith an optical disc surface 50 a, the stray light (7) (advancing path:minus-first-order diffraction light by diffraction→returning path:minus-first-order diffraction light by diffraction) is focused onto thelight receiving device.

When the focal position of the minus-first-order diffraction light iscloser to the objective lens 6 than to the optical disc surface 50 a,the stray light (5) (advancing path: minus-first-order diffraction lightby diffraction→returning path: zero-order diffraction light bytransmission) or the stray light (6) (advancing path: zero-orderdiffraction light by transmission→returning path: minus-first-orderdiffraction light by diffraction) can be focused as a small light spotonto the light receiving device under a predetermined condition.

When the objective lens 6 of this embodiment is used, as shown in FIG.19 (the enlarged view in the vicinity of the focal positions of FIG. 6),three points on which the laser beam is actually focused in the vicinityof an information recording surface 50 b of the first optical disc 50are the focal position (a) of the zero-order diffraction light, thefocal position (b) of the plus-first-order diffraction light, and thefocal position (c) of the minus-first-order diffraction light. The focalpositions (a) to (c) are conjugate points on the optical disc side ofthe “signal light B” (advancing path: zero-order diffraction light bytransmission→returning path: zero-order diffraction light bytransmission), the stray light (3) (advancing path: minus-first-orderdiffraction light by diffraction→returning path: plus-first-orderdiffraction light by diffraction), and the stray light (7) (advancingpath: minus-first-order diffraction light by diffraction→returning path:minus-first-order diffraction light by diffraction), which are focusedonto the light receiving device 9.

As described above, the conjugate point on the optical disc side of thestray light (5) (advancing path: minus-first-order diffraction light bydiffraction→returning path: zero-order diffraction light bytransmission) and the stray light (6) (advancing path: zero-orderdiffraction light by transmission→returning path: minus-first-orderdiffraction light by diffraction), focused as small light spots onto thelight receiving device 9, is the position of (d). Although the straylight (5) and the stray light (6) are not actually focused at theposition (d), they are focused as small light spots onto the lightreceiving device 9. Seen from the light receiving device 9, the position(d) can be regarded as an imaginary focal position.

Thus as shown in FIG. 19, when the focal position (a) of the zero-orderdiffraction light is matched with the information recording surface 50 bof the first optical disc 50 and the focal position (c) of theminus-first-order diffraction light is in the position as shown in FIG.19, the imaginary focal position (d) of the stray light (5) and thestray light (6) is matched with the surface 50 a of the first opticaldisc 50 and its reflection light is focused onto the light receivingdevice.

When the diffraction efficiencies of the zero-order diffraction light,the plus-first-order diffraction light, and the minus-first-orderdiffraction light are set as shown in FIG. 5, the focal spots of thestray light (5) and the stray light (6) have the amount of light largerthan that of the focal spot of the stray light (7). The stray light (5)and the stray light (6) pass in the substantially same optical path soas to be overlapped with each other on the light receiving device 9. Theratio of the amount of light of the stray light (5) and the stray light(6) to the signal light B as the light spot substantially used for writeor read is as much as about 30%. From the viewpoint of stabling theservo signal and the information signal, the influence of interferencebetween the stray lights and the signal lights cannot be neglected.

The minus-first-order diffraction light conjugating with theplus-first-order diffraction light is uniquely determined by the designof the plus-first-order diffraction light. Thus, the position relationbetween the plus-first-order diffraction light and the zero-orderdiffraction light need to be appropriately set in such a manner that thestray light due to the unnecessary diffraction light does not form asmall optical spot on the light receiving device. As described below,specifically, the hologram lens 5 and the objective lens 6 are designedso as to appropriately set a WD1 and a WD2 of the first optical disc 50and the second optical disc 60.

Here, with reference to FIG. 20, the design protective materialthickness of the first optical disc is t1, the design protectivematerial thickness of the second optical disc is t2, the workingdistance of the first optical disc is WD1, the working distance of thesecond optical disc is WD2, and the refractivity of the protectivematerial is n. The optical length of the protective material thickness thaving the refractivity n is t/n. As apparent from FIG. 20, thedifference (a converted length in the air) Δf0 between the focalposition of the plus-first-order diffraction light and the focalposition of the zero-order diffraction light is as follows:

since

(t1/n)+WD1+Δf0=(t2/n)+WD2

then

Δf0=((t2/n)+(WD2))−((t1/n)+(WD1))  (Equation 1)

The difference (a converted length in the air) Δf0′ between the focalposition of the minus-first-order diffraction light conjugating with theplus-first-order diffraction light and the focal position of thezero-order diffraction light will be considered as follows.

When the focal length of the objective lens 6 having the convex lenspower is f_o1 and the focal length of the plus-first-order diffractionlight of the hologram lens 5 having the concave lens power is f_doe(<0), the focal length of the minus-first-order diffraction lightconjugating with the plus-first-order diffraction light and having theconvex lens power is—f_doe (>0). The synthetic focal length fs of theplus-first-order diffraction light and the synthetic focal length fs_inv(minus-first-order diffraction light) of the minus-first-orderdiffraction light of the objective lens 6 and the hologram lens 5 areexpressed, with a lens interval d, by

fs=f _(—) o1×f _(—) doe/(f _(—) o1+f _(—) doe+d)  (Equation 2)

fs _(—) inv=−(f _(—) o1×f _(—) doe)/(f _(—) o1−f _(—) doe+d)  (Equation3)

The difference (the converted length in the air) Δf0 between the focalposition of the plus-first-order diffraction light and the focalposition of the zero-order diffraction light is not equal to thedifference (the converted length in the air) Δf0′ between the focalposition of the minus-first-order diffraction light and the focalposition of the zero-order diffraction light.

Δf0′=f0−k  (Equation 4)

(k is the focal position interval difference between theplus-first-order diffraction light and the minus-first-order diffractionlight uniquely determined by an optical system configured by theobjective lens 6 and the hologram lens 5)

Then,

Δf0′=(t2/n+(WD2))−(t1/n+(WD1))−k  (Equation 5)

Here, the signal light B (advancing path: zero-order diffraction lightby transmission→returning path: zero-order diffraction light bytransmission) for write or read of the optical disc having a smallprotective material thickness (the first optical disc 50) and the straylight (5) (advancing path: minus-first-order diffraction light bydiffraction→returning path: zero-order diffraction light bytransmission) or the stray light (6) (advancing path: zero-orderdiffraction light by transmission→returning path: minus-first-orderdiffraction light by diffraction) are overlapped with each other on thelight receiving device 9 when the substantially half of the differenceΔf′ in the focal position is matched with a converted length in the airΔd1_air of the interval between the information recording surface andthe surface of the optical disc.

When the first protective material thickness of the first optical discis t1_L1 and the second protective material thickness of the firstoptical disc is t1_L2, the converted length in the air Δd1_air_L1 andΔd1_air_L2 are expressed by

Δd1_air_(—) L1=t1_(—) L1/n  (Equation 6)

Δd1_air_(—) L2=t1_(—) L2/n  (Equation 7)

Each of the first protective material thickness and the secondprotective material thickness is an actual protective material thicknesswhen there are two information recording surfaces in one optical disc.Each of them includes a thickness error. The design protective materialthicknesses t1 and t2, as described above, are protective materialthicknesses used for designing the objective lens and have a conceptdifferent from that of the first protective material thickness and thesecond protective material thickness.

Noting Δf0′ shown in FIG. 19, when Δf0′/2 is matched with the firstprotective material thickness t1_L1 or the second protective materialthickness t1_L2, the signal light B and the stray light (5) or the straylight (6) are overlapped with each other on the light receiving device9. The overlapping conditions are expressed by the following equations10 and 11.

Δf0′=2×t1_(—) L1/n or Δf0′=2×t1_(—) L2/n

(t2/n+(WD2))−(t1/n+(WD1))−k=2×t1_(—) L1/n  (Equation 8)

(t2/n+(WD2))−(t1/n+(WD1))−k=2×t1_(—) L2/n  (Equation 9)

Then,

WD1−WD2=(t2−t1−(2×t1_(—) L1))/n−k  (Equation 10)

and

WD1−WD2=(t2−t1−(2×t1_(—) L2))/n−k  (Equation 11)

Here, a specific example is shown.

The design protective material thickness of the first optical disc ist1=0.0875 mm, the first protective material thickness of the firstoptical disc is t1_L1=0.1±0.005 mm (the first layer), the secondprotective material thickness of the first optical disc ist1_L2=0.075±0.005 mm (the second layer), the design protective materialthickness of the second optical disc is t2=0.6 mm, the refractivity ofthe protective material is n=1.6164, and the focal position intervaldifference of the plus-first-order diffraction light and theminus-first-order diffraction light is k=−0.03 mm.

The condition in which the signal light B and the stray light (5) or thestray light (6) are overlapped with each other on the light receivingdevice 9 is:

WD1−WD2=0.223±0.007 mm

or

WD1−WD2=0.254±0.007 mm

When the working distance WD1 of the first optical disc 50 is 0.5 mm,the working distance WD2 of the second optical disc 60 should not be setto near 0.277 mm and 0.246 mm.

Desirably,

WD1−WD2≠(t2t1−(2×t1_(—) L1))/n−k  (Equation 12)

and

WD1−WD2≠(t2t1−(2×t1_(—) L2))/n−k  (Equation 13)

The overlap of the signal light B with the stray light (5) or the straylight (6) on the light receiving device means that the focus errorsignals appear so as to be overlapped with each other.

FIG. 7 schematically shows the focus error signals (so-called S-shapedsignals) at write or read of the first optical disc 50 in the opticalhead 30 of this embodiment. In FIG. 7, the horizontal axis shows theamount of defocus, in which the right side shows the direction in whichthe objective lens 6 and the optical disc 50 are moved away from eachother and the left side shows the direction in which the objective lens6 and the optical disc 50 are brought close to each other. The verticalaxis shows the focus error signal level. The actual focus error signalsare asymmetric due to the influence of spherical aberration. Withoutconsidering it, FIG. 7 schematically shows the focus error signals.

In FIG. 7, A1 is the S-shaped signal obtained by reflection of thesignal light B on the information recording surface having a protectivematerial thickness of 0.100 mm, A2 is the S-shaped signal obtained byreflection of the signal light B on the information recording surfacehaving a protective material thickness of 0.075 mm, A3 is the S-shapedsignal obtained by reflection of the signal light B on the disc surface,B1 is the S-shaped signal obtained by reflection of the stray light (1)and the stray light (2) on the information recording surface having aprotective material thickness of 0.100 mm, B2 is the S-shaped signalobtained by reflection of the stray light (1) and the stray light (2) onthe information recording surface having a protective material thicknessof 0.075 mm, B3 is the S-shaped signal obtained by reflection of thestray light (1) and the stray light (2) on the disc surface, C1 is theS-shaped signal obtained by reflection of the stray light (5) and thestray light (6) on the information recording surface having a protectivematerial thickness of 0.100 mm, C2 is the S-shaped signal obtained byreflection of the stray light (5) and the stray light (6) on theinformation recording surface having a protective material thickness of0.075 mm, and C3 is the S-shaped signal obtained by reflection of thestray light (5) and the stray light (6) on the disc surface.

Noting one information recording surface or surface, as the objectivelens 6 is brought close to the disc, the S-shaped signals appear in theorder of:

-   -   the S-shaped signal obtained by reflection of the stray        light (1) and the stray light (2) the S-shaped signal obtained        by reflection of the signal light B    -   the S-shaped signal obtained by reflection of the stray        light (5) and the stray light (6)

The S-shaped signal by the stray light (3) and the stray light (4) issubstantially overlapped with the S-shaped signals (A1, A2, and A3) bythe signal light B, which is not a problem since the relative amount oflight of the stray lights (3) and (4) is small. The S-shaped signal bythe stray light (7) is not a problem since it has the small relativeamount of light and appears in the position farther from the signallight B than the S-shaped signals (C1, C2, and C3) by the stray light(5) and the stray light (6).

The signal light H (FIG. 5) becomes astray light for write or read ofthe first optical disc 50, which is not a problem since it has the smallrelative amount of light and appears in the position farther from thesignal light B than the S-shaped signals (B1, B2, and B3) by the straylight (1) and the stray light (2).

The above conditions

WD1−WD2≠(t2−t1−(2×t1_(—) L1))n−k  (Equation 12)

and

WD1−WD2≠(t2t1−(2×t1_(—) L2))/n−k  (Equation 13)

are conditions in which the zero cross points of the S-shaped signal A1,the S-shaped signal A2, and the S-shaped signal C3 are not matched witheach other. However, to sufficiently reduce the influence of theS-shaped signal C3 with respect to the S-shaped signal A1 and theS-shaped signal A2, the interval between the S-shaped signal A1 and theS-shaped signal C3 may be increased above the condition of the Equation12.

Specifically, the range in which the S-shaped signal A1 appears (e.g.,the range of above ±5 μm (converted in the air) with respect to the zerocross point of the S-shaped signal A1) and the range in which theS-shaped signal C3 appears (e.g., the range of above ±5 μm (converted inthe air) with respect to the zero cross point of the S-shaped signal C1)should not be overlapped with each other. When the amount of margin fornon-overlapping is X, (WD1−WD2) should be as follows.

WD1−WD2<(t2−t1−(2×t1_(—) L1))/n−k−X  (Equation 14)

When the amount of margin X is set in such a manner that the range ofabove ±5 μm (converted in the air) with respect to the zero cross pointof the S-shaped signal A1 and the range of above ±5 μm (converted in theair) with respect to the zero cross point of the S-shaped signal C1 arenot overlapped with each other, X>10 μm. It is desired that the amountof margin X with respect to the overlapped S-shaped signals beappropriately set according to an optical magnification.

For example, when X=0.01 mm, it is desired that WD1−WD2<0.207 mm. Forexample, when the working distance WD1 of the first optical disc 50 is0.5 mm, it is desired that the working distance WD2 of the secondoptical disc 60 be larger than 0.293 mm.

The amount of margin X is set as described above, so that the S-shapedsignal A1, the S-shaped signal A2, and other S-shaped signals (B1 to B3and C1 to C3) are completely separated from each other. Thus, theinfluence of the stray light with respect to the S-shaped signal A1 andthe S-shaped signal A2 can be substantially and sufficiently reduced.

As shown in FIG. 8, at read of the second optical disc 60, theminus-first-order diffraction light conjugating with theplus-first-order diffraction light having the concave lens power has theconvex lens power. Thus, the focal position of the minus-first-orderdiffraction light is closer to the objective lens 6 than the focalposition of the zero-order diffraction light. When the focal position ofthe minus-first-order diffraction light having the convex lens powerformed by the hologram lens 5 is matched with an optical disc surface 60a, the stray light (7) (advancing path: minus-first-order diffractionlight by diffraction→returning path: minus-first-order diffraction lightby diffraction) is focused onto the light receiving device 9.

FIG. 9 schematically shows the focus error signals (so-called S-shapedsignals) at read of the second optical disc 60 in the optical head 30 ofthis embodiment. In FIG. 9, the horizontal axis shows the amount ofdefocus, in which the right side shows the direction in which theobjective lens 6 and the optical disc 60 are moved away from each otherand the left side shows the direction in which the objective lens 6 andthe optical disc 60 are brought close to each other. The vertical axisshows the focus error signal level. The actual focus error signals areasymmetric due to the influence of spherical aberration. Withoutconsidering it, FIG. 9 schematically shows the focus error signals.

In FIG. 9, D1 is the S-shaped signal obtained by reflection of thesignal light H on the information recording surface having theprotective material thickness of 0.6 mm, D2 is the S-shaped signalobtained by reflection of the signal light H on the disc surface, E1 isthe S-shaped signal obtained by reflection of the stray light (1) andthe stray light (2) on the information recording surface having theprotective material thickness of 0.6 mm, E2 is the S-shaped signalobtained by reflection of the stray light (1) and the stray light (2) onthe disc surface, F1 is the S-shaped signal obtained by reflection ofthe signal light B on the information recording surface having theprotective material thickness of 0.6 mm, F2 is the S-shaped signalobtained by reflection of the signal light B on the disc surface, G1 isthe S-shaped signal obtained by reflection of the stray light (5) andthe stray light (6) on the information recording surface having theprotective material thickness of 0.6 mm, G2 is the S-shaped signalobtained by reflection of the stray light (5) and the stray light (6) onthe disc surface, H1 is the S-shaped signal obtained by reflection ofthe stray light (7) on the information recording surface having theprotective material thickness of 0.6 mm, and H2 is the S-shaped signalobtained by reflection of the stray light (7) on the disc surface.

Noting one information recording surface or disc surface, as theobjective lens 6 is brought close to the disc, the S-shaped signalsappear in the order of:

-   -   the S-shaped signal obtained by reflection of the signal light        H;    -   the S-shaped signal obtained by reflection of the stray        light (1) and the stray light (2);    -   the S-shaped signal obtained by reflection of the signal light        B;    -   the S-shaped signal obtained by reflection of the stray        light (5) and the stray light (6); and the S-shaped signal        obtained by reflection of the stray light (7).

(for the information recording surface, the S-shaped signals in thesolid line in FIG. 9)

The S-shaped signal of the stray light (3) and the stray light (4) issubstantially overlapped with the S-shaped signals (F1 and F2) of thesignal light B. The S-shaped signal G1 and the S-shaped signal H1 do notappear since the disc and the objective lens are not brought close toeach other any more (or collide with each other).

From FIG. 9, the interference between the S-shaped signal D1 and anotherS-shaped signal is a problem at read of the second optical disc 60. Toavoid this, the S-shaped signal D1 and the S-shaped signal H2 should notbe overlapped with each other. When the diffraction efficiencies of thezero-order diffraction light, the plus-first-order diffraction light,and the minus-first-order diffraction light are set as shown in FIG. 5,the ratio of the stray light (7) to the signal light H used for read is40% or above. Thus, from the viewpoint of stabling the servo signal andthe information signal, the interference between the S-shaped signal D1and the S-shaped signal H2 cannot be allowed.

As described above, the design protective material thickness of thefirst optical disc is t1, the design protective material thickness ofthe second optical disc is t2, the working distance of the first opticaldisc is WD1, the working distance of the second optical disc is WD2, andthe refractivity of the protective material is n. The difference Δf1 (aconverted length in the air) between the focal position of theplus-first-order diffraction light and the focal position of theminus-first-order diffraction light may be thought to be the sum of thedifference Δf0 (a converted length in the air) between the focalposition of the plus-first-order diffraction light and the focalposition of the zero-order diffraction light and the difference Δf0′ (aconverted length in the air) between the focal position of theminus-first-order diffraction light and the focal position of thezero-order diffraction light; that is, Δf1=Δf0+Δf0′

From Equation 4,

Δf1=2×Δf0−k

From Equation 1,

Δf1={(t2/n+WD2)−(t1/n+WD1)}×2−k  (Equation 15)

Here, the condition that the signal light H (advancing path:plus-first-order diffraction light by diffraction→returning path:plus-first-order diffraction light by diffraction) for read of theoptical disc having a large protective material thickness (the secondoptical disc 60) and the stray light (7) (advancing path:minus-first-order diffraction light by diffraction returning path:minus-first-order diffraction light by diffraction) are overlapped witheach other on the light receiving device is a case that the differenceΔf1 between the focal positions is matched with the converted length inthe air Δd2_air corresponding to the interval between the informationrecording surface and the surface of the optical disc.

When the protective material thickness of the second optical disc ist2_ra, the converted length in the air Δd2_air_ra is expressed by

Δd2_air_(—) ra=t2_(—) ra/n  (Equation 16)

The actual protective material thickness of the second optical disc isdifferent from that of the first optical disc. Regardless of the numberof the information recording surfaces of the disc, the range ofvariation in the protective material thickness of a first informationrecording surface and the range of variation in the protective materialthickness of a second information recording surface are substantiallymatched with each other and are both in the range of t2 ra. Accordingly,the condition for overlapping the signal light H and the stray light (7)with each other on the light receiving device is a case, as apparentfrom FIG. 21, when Δf1 is matched with t2_ra/n (=Δd2_air_ra). So,

{(t2/n+(WD2))−(t1/n+(WD1))}×2−k=t2_(—) ra/n  (Equation 18)

then

WD1−WD2=(2×t2−2×t1−t2_(—) ra)/(2×n)−k  (Equation 20)

Here, the design protective material thickness of the first optical discis t1=0.0875 mm, the design protective material thickness of the secondoptical disc is t2=0.6 mm, the protective material thickness of thesecond optical disc is 0.57≦t2_ra≦0.63 mm, the refractivity of theprotective material is n=1.6164, and the focal position intervaldifference between the plus-first-diffraction light and theminus-first-order diffraction light is k=−0.03 mm. Under the above case,the condition in which the signal light H and the stray light (7) areoverlapped with each other on the light receiving device is:

0.152 mm≦WD1−WD2≦0.171 mm

When the working distance WD1 of the first optical disc 50 is 0.5 mm,the working distance WD2 of the second optical disc 60 should not be setto 0.329 mm≦WD2≦0.348 mm.

Desirably,

WD1−WD2≠(2×t2−2×t1−t1_(—) ra)/(2×n)−k  (Equation 22)

These conditions are conditions in which the zero cross points of theS-shaped signal D1 and the S-shaped signal H2 are not matched with eachother. However, to sufficiently reduce the influence of the S-shapedsignal H2 with respect to the S-shaped signal D1, it is desired that theinterval between the S-shaped signal A1 and the S-shaped signal C3 bedecreased above the condition of Equation 23.

Specifically, the range in which the S-shaped signal D1 appears (e.g.,the range of above ±5 μm converted in the air with respect to the zerocross point of the S-shaped signal D1) and the range in which theS-shaped signal H2 appears (e.g., the range of above ±5 μm converted inthe air with respect to the zero cross point of the S-shaped signal H2)should not be overlapped with each other. When the amount of margin isX′, it should be:

WD1−WD2>(2×t2−2×t1−(t1_(—) ra))/(2×n)−k+X′  (Equation 24)

When the amount of margin X′ is set in such a manner that the range ofabove ±5 μm converted in the air with respect to the zero cross point ofthe S-shaped signal D1 and the range of above ±5 μm converted in the airwith respect to the zero cross point of the S-shaped signal H2 are notoverlapped with each other, X′≧10 μm. It is desired that the amount ofmargin X′ with respect to the overlapped S-shaped signals beappropriately set according to an optical magnification.

For example, when X′=0.01 mm, it is desired that WD1−WD2>0.181 mm. Forexample, when the working distance WD1 of the first optical disc 50 is0.5 mm, it is desired that the working distance WD2 of the secondoptical disc 60 be smaller than 0.319 mm.

By setting as described above, the S-shaped signal D1 and other S-shapedsignals (D2 to H2) are completely separated from each other. Thus, theinfluence of the stray light on the S-shaped signal D1 can besubstantially and sufficiently reduced.

When the second optical disc 60 has a plurality of information recordingsurfaces, a plurality of S-shaped signals according to the number of theinformation recording surfaces appear for the S-shaped signals D1, E1,F1, G1, and H1 in FIG. 9. Even when the second optical disc 60 has aplurality of information recording surfaces, the position relationbetween each of the S-shaped signals and other S-shaped signals is notchanged. Accordingly, in this embodiment, the number of the informationrecording surfaces of the second optical disc 60 is not particularlydescribed. The present invention is applicable to the second opticaldisc 60 having a plurality of information recording surfaces.

Embodiment 2

FIGS. 10 and 11 are schematic block diagrams of an optical head 40according to another embodiment of the present invention.

In FIGS. 10 and 11, the same components as those of Embodiment 1 areindicated by similar reference numerals and the description will beomitted below.

In FIGS. 10 and 11, the optical head 40 includes a light source 1emitting a blue-violet laser beam, a beam splitter 3, a collimate lens4, a hologram integrated type objective lens 16, a detecting lens 8, anda light receiving device 9 for receiving the laser beam. The referencenumeral 50 denotes a first optical disc as an optical disc having aprotective material with a thickness of about 0.075 to 0.1 mm. Thereference numeral 60 denotes a second optical disc as an optical dischaving a protective material with a thickness of about 0.6 mm.

The operation of the optical head 40 which writes or reads informationto/from the first optical disc 50 will be described using FIG. 10. Theblue-violet laser beam emitted from the light source 1 passes throughthe beam splitter 3 and is then converted to a substantially parallelbeam by the collimate lens 4. The blue-violet laser beam is focused as alight spot onto the information recording surface of the first opticaldisc 50 beyond the protective material by the hologram integrated typeobjective lens 16. The blue-violet laser beam in a returning pathreflected on the information recording surface of the first optical disc50 passes through the hologram integrated type objective lens 16 and thecollimate lens 4 in the same optical path as an advancing path. Theblue-violet laser beam in the returning path is reflected by the beamsplitter 3 and is then given predetermined astigmatism by the detectinglens 8. The blue-violet laser beam is led to the light receiving device9 so as to generate an information signal and a servo signal.

The operation of the optical head 40 which writes or reads informationto/from the second optical disc 60 will be described with reference toFIG. 11. The blue-violet laser beam emitted from the light source 1passes through the beam splitter 3 and is then converted to asubstantially parallel beam by the collimate lens 4. The blue-violetlaser beam is diffracted and focused by the hologram integrated typeobjective lens 16 and is then focused as a light spot onto theinformation recording surface of the second optical disc 60 beyond theprotective material. The blue-violet laser beam in a returning pathreflected on the information recording surface of the second opticaldisc 60 passes through the hologram integrated type objective lens 16and the collimate lens 4 in the same optical path as an advancing path.The blue-violet laser beam in the returning path is reflected by thebeam splitter 3 and is then given predetermined astigmatism by thedetecting lens 8. The blue-violet laser beam is led to the lightreceiving device 9 so as to generate an information signal and a servosignal.

The function of the hologram integrated type objective lens 16 will bedescribed in detail with reference to FIGS. 12 and 13. As in Embodiment1, the hologram integrated type objective lens 16 can be driven by theactuator 31 shown in FIG. 18 by control of the control unit 402 in thefocus direction 31 a and the tracking direction 31 b.

The hologram integrated type objective lens 16 is formed with aconcentric grating pattern 16 a. Its center is matched with the opticalaxis of the hologram integrated type objective lens 16. The gratingpattern 16 a is formed only in the portion having a diameter smallerthan an aperture determined by the hologram integrated type objectivelens 16. No diffraction occurs in the portion not formed with thegrating pattern 16 a at all.

The diffraction efficiency of the plus-first-order diffraction light ofthe hologram integrated type objective lens 16 is less than 100%. Thehologram integrated type objective lens 16 is designed in such a mannerthat the transmission light (zero-order diffraction light) hassufficient intensity. The hologram integrated type objective lens 16 isblazed so as to increase the sum of the amount of the zero-orderdiffraction light and the plus-first-order diffraction light. Thus thelight utilization efficiency can be high.

When the hologram integrated type objective lens 16 is used as theoptical head only for read of both the first optical disc 50 and thesecond optical disc 60, it is desired that the diffraction efficiency ofthe plus-first-order diffraction light be about 30 to 70%. By settingthe diffraction efficiency in this manner, the first optical disc 50 andthe second optical disc 60 can read information using a substantiallyequal amount of light. Thus the output of the light source can bereduced.

When the hologram integrated type objective lens 16 is used as theoptical head which performs only read of the second optical disc 60 andcan perform write and read of the first optical disc 50, it is desiredthat the diffraction efficiency of the plus-first-order diffractionlight be 30% or below. By setting the diffraction efficiency in thismanner, the transmittance (the diffraction efficiency of the zero-orderdiffraction light) of the hologram integrated type objective lens 16 canbe increased. Thus the light utilization efficiency with respect to thefirst optical disc 50 for write can be high. So, the output of the lightsource at write can be reduced.

The hologram integrated type objective lens 16 has an NA of 0.85 and isdesigned so as to form a focal spot onto the first optical disc 50having a protective material thickness of 0.1 mm to diffraction limit,as shown in FIG. 13A, when the laser beam passed through the hologramintegrated type objective lens 16 without being diffracted (that is, thezero-order diffraction light) is incident.

As shown in FIG. 13B, the plus-first-order diffraction light diffractedby the hologram integrated type objective lens 16 is focused onto thesecond optical disc 60. Here, the plus-first-order diffraction light isaberration-corrected so as to form a focal spot onto the second opticaldisc 60 having a protective material thickness of about 0.6 mm todiffraction limit.

The hologram integrated type objective lens 16 for diffracting part ofthe incident light can realize a bifocal lens which can form a focalspot focused onto each of the optical discs having different substratethicknesses to diffraction limit.

The hologram integrated type objective lens 16 of this embodiment hasthe effect of adding the concave lens power to the plus-first-orderdiffraction light with respect to the zero-order diffraction light.Thus, the focal position of the plus-first-order diffraction light withrespect to the hologram integrated type objective lens 16 is fartherthan that of the zero-order diffraction light. Accordingly, when theplus-first-order diffraction light is focused onto the informationrecording surface of the second optical disc 60 having a protectivematerial thickness larger than that of the first optical disc 50, aworking distance (WD) as the interval between the second optical disc 60and the hologram integrated type objective lens 16 can be sufficientlysecured.

The hologram integrated type objective lens 16 provides the concave lensaction to the plus-first-order diffraction light. So, the positions inthe optical axis direction of two focal points of the zero-orderdiffraction light for write or read of the first optical disc 50 and theplus-first-order diffraction light for write or read of the secondoptical disc 60 are different. Thus when information is written or readusing a light spot formed on one of the focal points, a light spotfocused on the other focal point is enlarged, which does not affectwrite or read of information.

When the grating pattern is formed so as to generate the desiredplus-first-order diffraction light by the hologram integrated typeobjective lens 16, in addition to the zero-order diffraction light andthe plus-first-order diffraction light, the minus-first-orderdiffraction light conjugating with the plus-first-order diffractionlight and a higher order diffraction light are generated. As inEmbodiment 1, the relation between the plus-first-order diffractionlight and the zero-order diffraction light need to be appropriately setin such a manner that the stray light due to the unnecessary diffractionlight does not form a small light spot on the light receiving device 9.As in Embodiment 1, the hologram lens 5 and the objective lens 6 aredesigned so as to appropriately set the WD1 and WD2 for the firstoptical disc 50 and the second optical disc 60.

As shown in Embodiment 1, the design protective material thickness ofthe first optical disc is t1, the design protective material thicknessof the second optical disc is t2, the working distance of the firstoptical disc is WD1, the working distance of the second optical disc isWD2, the refractivity of the protective material is n, the firstprotective material thickness of the first optical disc is t1_L1, andthe second protective material thickness of the first optical disc ist1_L2.

In the above condition, the conditions in which the signal light B andthe stray light (5) or the stray light (6) shown in FIG. 5 areoverlapped with each other on the light receiving device 9 are asfollows:

from

(t2/n+(WD2))−(t1/n+(WD1))−k=2×t1_(—) L1/n  (Equation 8)

(t2/n+(WD2))−(t1/n+(WD1))−k=2×t1_(—) L2/n  (Equation 9)

then

WD1−WD2=(t2−t1−(2×t1_(—) L1))/n−k  (Equation 10)

and

WD1−WD2=(t2−t1−(2×t1_(—) L2))/n−k  (Equation 11)

Here, the design protective material thickness of the first optical discis t1=0.0875 mm, the first protective material thickness of the firstoptical disc is t1_L1=0.1±0.005 mm (the first layer), the secondprotective material thickness of the first optical disc ist1_L2=0.075±0.005 mm (the second layer), the design protective materialthickness of the second optical disc is t2=0.6 mm, the refractivity ofthe protective material is n=1.6164, the focal position intervaldifference between the plus-first-order diffraction light and theminus-first-order diffraction light is k=0.03 mm.

In the above condition, the condition in which the signal light B andthe stray light (5) or the stray light (6) are overlapped with eachother on the light receiving device is:

WD1−WD2=0.163±0.007 mm

or

WD1−WD2=0.194±0.007 mm

When the working distance WD1 of the first optical disc is 0.5 mm, theworking distance WD2 of the second optical disc should not be set tonear 0.337 mm and 0.306 mm.

Desirably,

WD1−WD2≠(t2−t1−(2×t1_(—) L1))/n−k  (Equation 12)

WD1−WD2≠(t2t1−(2×t1_(—) L2))/n−k  (Equation 13)

Further, desirably,

WD1−WD2<(t2−t1−(2×t1_(—) L))n−k−X  (Equation 14)

When X=0.01 mm, it is desired that WD1−WD2<0.147 mm. The X is the amountof margin described in Embodiment 1.

For example, when the working distance of the first optical disc 50 isset to 0.5 mm, the working distance WD2 of the second optical disc 60 isdesirably larger than 0.353 mm.

By setting those conditions as described above, the S-shaped signal A1,the S-shaped signal A2, and other S-shaped signals (B1 to B3 and C1 toC3) in FIG. 7 are completely separated from each other. The influence ofthe stray light with respect to the S-shaped signal A1 and the S-shapedsignal A2 can be substantially and sufficiently reduced.

At read of the second optical disc 60, when the protective materialthickness of the second optical disc is t2_ra, the condition in whichthe signal light H and the stray light (7) shown in FIG. 5 areoverlapped with each other on the light receiving device is as follows:

from

{(t2/n+WD2)−(t1/n+WD1)}×2−k=t2_(—) ra/n  (Equation 18)

then

WD1−WD2=(2×t2−2×t1−t2_(—) ra)/(2×n)−k  (Equation 20)

Here, the design protective material thickness of the first optical discis t1=0.0875 mm, the design protective material thickness of the secondoptical disc is t2=0.6 mm, the protective material thickness of thesecond optical disc is 0.57 mm≦t2_ra≦0.63 mm, the refractivity of theprotective material is n=1.6164, and the focal position intervaldifference between the plus-first-diffraction light and theminus-first-order diffraction light is k=0.03 mm. In the abovecondition, the condition in which the signal light H and the stray light(7) are overlapped with each other on the light receiving device is:

0.092 mm≦WD1−WD2≦0.111 mm

So, when the working distance WD1 of the first optical disc 50 is 0.5mm, the working distance WD2 of the second optical disc 60 should not beset to 0.389 mm≦WD2≦0.408 mm.

Desirably,

WD1−WD2≠(2×t2−2×t1−t2_(—) ra)/(2×n)−k  (Equation 22)

Further, desirably,

WD1−WD2>(2×t2−2×t1−t2_(—) ra)/(2×n)−k+X′  (Equation 24)

For example, when X′=0.01 mm, it is desired that WD1−WD2>0.121 mm. TheX′ is the amount of margin described in Embodiment 1.

For example, when the working distance WD1 of the first optical disc 50is 0.5 mm, it is desired that the working distance WD2 of the secondoptical disc 60 be smaller than 0.379 mm.

By setting those conditions as described above, the S-shaped signal D1and other S-shaped signals (D2 to H2) are completely separated from eachother. Thus the influence of the stray light with respect to theS-shaped signal D1 can be substantially and sufficiently reduced.

In Embodiments 1 and 2, the condition in which the signal light and thestray light are not overlapped with each other on the light receivingdevice 9 is explained in detail in the case where, in the first opticaldisc 50, each of the information recording surface indicated by thefirst protective material thickness t1_L1 and the information recordingsurface indicated by the second protective material thickness t1_L2 hasan error of a predetermined protective material thickness, and in thesecond optical disc 60, the information recording surface indicated bythe protective material thickness t2_ra has an error of a predeterminedprotective material thickness, not depending on the number ofinformation recording surfaces.

However, the present invention is not limited to such Embodiments and isapplicable to the case that the first optical disc 50 has three or moreinformation recording surfaces and each of them has an error of apredetermined protective material thickness and that the second opticaldisc 60 has an error of a predetermined protective material thicknessfor each information recording surface.

Namely, the idea of the present invention is that with respect to therange in which the protective material thickness of the optical disc canbe obtained, the signal light reflected on the information recordingsurface and the stray light reflected on the surface of the optical discclose to the objective lens are not overlapped with each other on thelight receiving device 9. Needless to say, the present invention iswidely applicable to the optical head having the diffraction device forgenerating diffraction lights of a plurality of orders.

In Embodiments 1 and 2, the zero-order diffraction light is used forwrite or read of the first optical disc 50 and the plus-first-orderdiffraction light is used for write or read of the second optical disc60. However, the present invention is applicable when theplus-first-order diffraction light is used for write or read of thefirst optical disc 50 and the zero-order diffraction light is used forwrite or read of the second optical disc 60. In this case, as theplus-first-order diffraction light for write or read of the firstoptical disc 50 has the convex lens power, it has the merit of cancelingchromatic aberration caused by the objective lens.

The hologram device and the hologram integrated type objective lens inthis case form a first grating pattern only in the portion having adiameter smaller than the aperture for the first optical disc and asecond grating pattern outside of it. Here, it is desired that thehologram device and the hologram integrated type objective lens bedesigned in such a manner that the diffraction efficiency(transmittance) of the zero-order diffraction light in the first gratingpattern is less than 100%, the plus-first-order diffraction light hassufficient intensity, and the diffraction efficiency of theplus-first-order diffraction light in the second grating pattern ismaximum.

When the plus-first-order diffraction light is used for write or read ofthe first optical disc 50 and the zero-order diffraction light is usedfor write or read of the second optical disc 60, the stray light due toa plus-second-order or higher order diffraction light and aminus-second-order or higher order diffraction light (e.g., advancingpath: plus-first-order diffraction light by diffraction→returning path:plus-second-order diffraction light by diffraction) need to beconsidered.

The present invention is applicable to the optical head compatible withplural kinds of optical discs using a combination of the zero-orderdiffraction light and the plus-second-order diffraction light, theplus-first-order diffraction light and the plus-second-order diffractionlight, or higher order diffraction lights. It is possible to realize theconfiguration of the optical head in such a manner that the stray lightdue to the necessary diffraction light does not form a small light spoton the light receiving device 9.

It is apparent that the influence of the stray light due to thenecessary diffraction light is increased when the ratio of thereflectivity of the optical disc surface to the information recordingsurface of the optical disc is large. It is effective for theinformation recording medium satisfying:

0.5≦Rr/Rs≦3.0  (Equation 25)

where a reflectivity of the information recording surface is Rr and asurface reflectivity is Rs. Such information recording medium has two ormore information recording surfaces. It is particularly effective forthe optical head for write or read of such information recording medium.

In Embodiments 1 and 2, the optical head of an infinite optical systemhaving the collimate lens, that is, the condensing lens, is described.However, the present invention is not limited to this and is apparentlyapplicable to an optical head of a finite optical system not having thecollimate lens, that is, the condensing lens. In such optical head ofthe finite optical system, an objective lens functions as an objectivelens and a condensing lens.

In Embodiments 1 and 2, there are described in detail the condition inwhich the stray light (5) (advancing path: minus-first-order diffractionlight by diffraction→returning path: zero-order diffraction light bytransmission) or the stray light (6) (advancing path: zero-orderdiffraction light by transmission→returning path: minus-first-orderdiffraction light by diffraction), shown in FIG. 5 and reflected on theoptical disc surface of the first optical disc 50, is focused onto thelight receiving device 9 and the condition in which the stray light (7)(advancing path: minus-first-order diffraction light by diffraction Xreturning path: minus-first-order diffraction light by diffraction)reflected on the optical disc surface of the second optical disc 60 isfocused onto the light receiving device 9.

The stray lights (5) to (7) are not a problem when the concave lenspower of the plus-first-order diffraction light is very large or verysmall.

When the concave lens power of the plus-first-order diffraction light islarge, the convex lens power of the minus-first-order diffraction lightconjugating with the plus-first-order diffraction light is increased. Inthis case, the interval between the focal positions of theplus-first-order diffraction light, the zero-order diffraction light,and the minus-first-order diffraction light is increased. So, as shownin FIG. 22, the focal position of the minus-first-order diffractionlight is moved away from the optical disc surface 50 a and the straylights (5), (6), and (7) are not focused onto the light receiving device9. Namely, this is not substantially a problem.

However, to increase the concave lens power of the plus-first-orderdiffraction light, the pitch of the grating pattern of the hologram lens5 need to be small. So, the difficulty of molding the hologram lens isincreased and the cost becomes high. The lower limit of the gratingpitch is substantially 2 μm. Thus, such hologram lens cannot be actuallyemployed.

When the concave lens power of the plus-first-order diffraction light issmall, the convex lens power of the minus-first-order diffraction lightconjugating with the plus-first-order diffraction light is decreased.The interval between the focal positions of the plus-first-orderdiffraction light, the zero-order diffraction light, and theminus-first-order diffraction light is decreased. So, as shown in FIG.23, the focal position of the higher order diffraction light, e.g., theminus-second-order diffraction light or a minus-third-order diffractionlight is matched with the optical disc surface 60 a. The diffractionefficiency of the higher order diffraction lights is smaller than thatof the plus-first-order diffraction light, the minus-first-orderdiffraction light, and the zero-order diffraction light. Thus, when thestray lights reflected on the optical disc surface 60 a are focused ontothe light receiving device 9, the influence is very small.

However, it is impossible to neglect the influence of the light spot inwhich the zero-order diffraction light having large diffractionefficiency is reflected on the information recording surface, not on theoptical disc surface, so as to be focused onto the light receivingdevice 9.

As described above, the present invention is not limited to the opticalheads 30 and 40 described in Embodiments 1 and 2 and is widelyapplicable to an optical head in which a position in which a diffractionlight with the n-order diffraction light in the advancing path and then-order diffraction light in the returning path, reflected on theinformation recording surface of the optical disc, is focused onto thelight receiving device 9 by the condensing lens is different from aposition in which a diffraction light different from the n-orderdiffraction light in at least one of the advancing path and thereturning path, reflected on the surface of the optical disc, is focusedonto the light receiving device 9 by the condensing lens.

In particular, the present invention is preferably applied to an opticalhead for defining the lens power of the diffraction light in such amanner that the stray light (5) (advancing path: minus-first-orderdiffraction light by diffraction→returning path: zero-order diffractionlight by transmission) or the stray light (6) (advancing path:zero-order diffraction light by transmission→returning path:minus-first-order diffraction light by diffraction), reflected on thesurface of the first optical disc, is not focused onto the lightreceiving device or that the stray light (7) (advancing path:minus-first-order diffraction light by diffraction→returning path:minus-first-order diffraction light by diffraction) reflected on theoptical disc surface of the second optical disc is not focused onto thelight receiving device.

Embodiment 3

In Embodiments 1 and 2, there is described the optical head in which forthe objective lens for focusing the n-order diffraction light and them-order diffraction light having the same wavelength onto theinformation recording surfaces of different kinds of optical discs, theposition in which a diffraction light with the n-order diffraction lightin the advancing path and the n-order diffraction light in the returningpath, reflected on the information recording surface of the opticaldisc, is focused onto the light receiving device by the condensing lensis different from the position in which a diffraction light differentfrom the n-order diffraction light in at least one of the advancing pathand the returning path, reflected on the surface of the optical disc, isfocused onto the light receiving device by the condensing lens. Thepresent invention is not limited to these embodiments.

The present invention is applicable to an optical head having anobjective lens for diffracting laser beams having different wavelengthsso as to focus the laser beams onto information recording surfaces ofdifferent kinds of optical discs.

A diffraction angle θ expressing the power of the hologram lens isdetermined by

n·λ=d·sin θ

where n is a diffraction order, k is a wavelength, and d is a gratingpitch.

The d is a set value to the lens. So, when the laser beam is focusedonto information recording surfaces of different kinds of optical discs,at least one of the diffraction order n and the wavelength λ may bechanged.

When the optical head compatible with different kinds of optical discsis realized, use of diffraction light having the same wavelength anddifferent order, use of diffraction lights having the same order anddifferent wavelengths, or use of diffraction lights having differentorders and different wavelengths, is a design matter. The optical headis designed in such a manner that the position in which a diffractionlight having the n-order diffraction light in the advancing path and then-order diffraction light in the returning path, reflected on theinformation recording surface of the predetermined information recordingmedium, is focused onto the light receiving device by the condensinglens is different from the position in which a diffraction lightdifferent from the n-order diffraction light in at least one of theadvancing path and the returning path, reflected on the surface of theinformation recording surface, is focused onto the light receivingdevice by the condensing lens. Each case is effective and is in therange in which the present invention is applicable.

The present invention applied to such optical head will be described indetail with reference to FIGS. 24 to 27.

An objective lens 26 shown in FIGS. 24 to 27 has a diffraction structurewhich uses the difference between wavelengths so as to focus theplus-third-order diffraction light of the blue-violet laser beam ontothe information recording surface of the BD 50 having a protectivematerial thickness of 0.1 mm, to focus the plus-second-order diffractionlight of the red laser beam onto the information recording surface ofthe DVD 70 having a protective material thickness of 0.6 mm, and tofocus the plus-second-order diffraction light of the infrared laser beamonto the information recording surface of the CD 80 having a protectivematerial thickness of 1.2 mm. The objective lens 26 is designed in sucha manner that the diffraction efficiency of the plus-third-orderdiffraction light of the blue-violet laser beam is about 93%, thediffraction efficiency of the plus-second-order diffraction light of thered laser beam is about 94%, and the diffraction efficiency of theplus-second-order diffraction light of the infrared laser beam is about52%.

The objective lens 26 has a diffraction structure in which theplus-first-diffraction light has the concave lens power in order tofocus the plus-first-order diffraction light having a correspondingwavelength onto the information recording surfaces of the BD 50, the DVD70, and the CD 80. This structure is employed in order that as thewavelength is longer, the concave lens power is increased because thered laser beam and the infrared laser beam having a long wavelength areused so as to be focused onto the information recording surfaces of theDVD 70 and the CD 80 having a large protective material thickness.

As shown in FIG. 27, in the infrared laser beam, the focal position ofthe plus-first-order diffraction light having a concave lens powersmaller than that of the plus-second-order diffraction light is in thedirection close to the optical disc surface. The diffraction efficiencyof the plus-first-order diffraction light is about 29%. So, the concavelens power is preferably determined in such a manner that theplus-first-order diffraction light is reflected on the surface of the CD80 so as not to be focused onto the light receiving device 9 by thecondensing lens.

Embodiment 4

FIG. 14 is a schematic block diagram of an optical disc apparatusaccording to an embodiment of the present invention.

In FIG. 14, the reference numeral 400 denotes the optical disc apparatuswhich has in its inside an optical disc driving unit 401, a control unit402, and an optical head 403. The reference numeral 50 denotes the firstoptical disc which can be replaced with the second optical disc 60.

The optical disc driving unit 401 has the function of rotating anddriving the first optical disc 50 or the second optical disc 60. Theoptical head 403 is any one of the optical heads described inEmbodiments 1 to 3. The control unit 402 has the function of driving andcontrolling the optical disc driving unit 401 and the optical head 403,the function of performing signal processing of a control signal and aninformation signal light-received by the optical head 403, and thefunction of interfacing the information signal with the outside andinside of the optical disc apparatus 400.

The optical disc apparatus 400 is equipped with any one of the opticalheads described in Embodiments 1 to 3. So, the optical disc apparatus400 of this embodiment can satisfactorily perform write or read of aplurality of optical discs.

A focus control method preferable for the optical head 403 as any one ofthe optical heads described in Embodiments 1 to 3 executed by thecontrol unit 402 in the optical disc apparatus 400 will be describedbelow. The focus control method is not limited to the optical headsdescribed in Embodiments 1 to 3.

To write information onto the information recording surface of theoptical disc or read the information written onto the informationrecording surface of the optical disc, it is necessary to control theposition of the objective lens in the optical axis direction so as tofocus a focal spot of the laser beam passing through the objective lensonto the information recording surface. This is called focus control.The focus control can be realized by a control loop which detects thefocus error signal indicating that how far a focal spot is away from theinformation recording surface, that is, the amount of a focal error, bythe optical head and moves the objective lens in the direction reducingthe focus error signal. Operating the control loop is referred to as“starting focus control”. As described in Embodiment 1, an unit portionhaving the objective lens is driven by the actuator 31 in the focusdirection 31 a so as to perform the focus control.

The range in which the focus error signal is substantially proportionalto the amount of a focal error is limited to the case that the focalerror is in a certain fixed range. The range is called a controllablerange. The amount is ±10 μm at most and can also be +1 μm, depending onthe design. It is necessary to start the focus control when the amountof a focal error becomes sufficiently small so as to be in thecontrollable range while the objective lens is moved by the actuator 31in the optical axis direction. An embodiment of such focus controlstarting method will be described with reference to FIGS. 28 to 32. Itis preferable that an integrated circuit storing such focus controlstarting method be used as an electric circuit used for the optical discapparatus 400.

FIG. 28 shows an example of the algorithm of the focus control startingmethod. For the optical head 403 in the optical disc apparatus 400, theoptical head 30 described in Embodiment 1 is taken as an example.

When the optical disc apparatus 400 is turned on or the optical disc,e.g., one of the optical disc 50 and the optical disc 60 described inEmbodiments 1 and 2, is inserted into the optical disc apparatus 400, aread instruction is given from the electric circuit (step S1). In stepS2, the objective lens 6 is moved in the direction close to the opticaldisc while the light source 1 is worked. To bring the objective lens 6close to the optical disc, a focus driving current is flowed to theactuator 31. The amount of the focus driving current is increased ordecreased in a constant amount so as to move the objective lens 6 in theoptical axis direction. Desirably, the sensitivity (focus sensitivity)of the actuator 31 is designed or measured at designing or manufacturingstep, the amount of the focus driving current increased or decreased isdetermined in such a manner that the objective lens 6 is not broughtinto contact with the optical disc surface, and the amount of movementis stored in the memory unit in an information device such as a flashmemory.

When the objective lens 6 is brought close to the optical disc, thefocus error signal is changed. When the thickness of the transparentprotective material of the optical disc is small, e.g., 0.1 mm, and asshown in FIG. 29, the zero-order diffraction light is focused onto theinformation recording surface, the spherical aberration is equal to orlower than 70 m λ rms, that is, the Marechal criterion. In the state asdesigned, a focal spot can be obtained to diffraction limit. The lightfor forming the focal spot is reflected on the information recordingsurface and then passes through the hologram lens 5 again. The focuserror signals formed by incidence of the zero-order diffraction lightcaused at this time on the light receiving device 9 are A1 and A2 shownin FIG. 30. FIG. 30 shows a two-layer optical disc having twoinformation recording surfaces. The A2 is the focus error signalcorresponding to the information recording surface present in theposition in which the light passes through the transparent protectivematerial having a thickness of about 0.1 mm from the surface of theoptical disc. When the diffraction efficiency (transmission efficiency)of the zero-order diffraction light of the hologram lens 5 is about 65%and the diffraction efficiency of the plus-first-order diffraction lightis about 15%, the intensity of the zero-order diffraction light is largeand the aberration is minimum. Thus, the A1 and A2 form significantlylarge signals.

When the large focus error signal appears in the position at a distanceof about 0.1 mm from the surface, the optical disc inserted into theoptical disc apparatus 400 can be discriminated as the optical dischaving a transparent protective material thickness of about 0.1 mm. Athreshold value LB is provided, and the objective lens 6 is broughtclose to the optical disc from the position away from the optical discagain so as to start the focus control when the focus error signalexceeds the threshold value LB.

When the optical disc inserted into the optical disc apparatus 400 isthe optical disc having a transparent protective material thickness ofabout 0.1 mm, the focus control can be started immediately.

The threshold value LB is the isolation amount of the focus error signalfrom the reference signal level and can be positive or negative. Themagnitude of the threshold value is defined as an absolute value of theisolation amount of the focus error signal from the reference signallevel. In step S2, the amount of light PS incident on the lightreceiving device 9 is monitored so as to measure the maximum value FSM.By measuring the FSM, the focus error signal is standardized by the FSM.Although the reflectivity of the information recording surface of theoptical disc is often different for each of the optical discs,regardless of it, the focus error signal of the same shape can beobserved.

When the large focus error signal does not appear in the position at thedistance of about 0.1 mm from the optical disc surface, the optical disccan be the optical disc having a transparent protective materialthickness of 0.6 mm. In such case, in step S3, as the objective lens 6need to be brought closer to the optical disc, the operation forobtaining the sensitivity of the actuator 31 is performed. Thesensitivity of the actuator 31 is an amount of movement of the objectivelens 6 per unit current flowed to the actuator 31. As the amount of anelectric current can be measured by a voltage generated in particularresistance, the sensitivity of the actuator 31 can also be defined asper unit voltage. In the following description, the sensitivity of theactuator 31 can also be called a focus sensitivity. For the sensitivityof the actuator 31, at manufacturing the optical head, the actuator 31is actually moved so as to measure the amount of movement by a laserdistance measuring device and the result is stored in a nonvolatilememory such as a flash memory. In this step, the focus sensitivity ofthe actuator may be obtained from the memory.

To obtain a higher accuracy focus sensitivity, the following method iseffective. When the transparent protective material thickness of theoptical disc is larger, e.g., 0.6 mm, and as shown in FIG. 31, when theplus-first-diffraction light of the hologram lens 5 is focused onto theinformation recording surface of the optical disc, the sphericalaberration is equal to or lower than 70 m λ rms, that is, the Marechalcriterion. In the state as designed, a focal spot can be obtained todiffraction limit. The light for forming the focal spot is reflected onthe information recording surface and then passes through the hologramlens 5 again. The focus error signal formed by incidence of theplus-first-order diffraction light generated at this time on the lightreceiving device 9 is D1 shown in FIG. 32. FIG. 32 shows a single-layeroptical disc having one information recording surface.

A threshold value LC is provided and the objective lens 6 is broughtclose to the optical disc from the position away from the optical discagain so as to monitor the amount of the actuator driving current or thedifference between the voltages from the first point at which the focuserror signal exceeds the threshold value LC to the next point at whichthe focus error signal exceeds the threshold value LC. The first pointat which the focus error signal exceeds the threshold value LC is thesignal E2. This is a signal which overlaps the signals obtained in sucha manner that a laser beam having the zero-order laser beam and theplus-first-order laser beam and a laser beam having the plus-first-orderlaser beam and the zero-order laser beam in the advancing path and thereturning path in the hologram lens 5 are combined and incident on thelight receiving device 9. The next point at which the focus error signalexceeds the threshold value LC is the signal F2. This is a signalobtained in such a manner that the zero-order laser beam in theadvancing path and the zero-order laser beam in the returning path isincident on the light receiving device 9. A further point at which thefocus error signal exceeds the threshold value LC is the signal G2. Thisis a signal which overlaps the signals obtained in such a manner that alaser beam having the zero-order laser beam and the minus-first-orderlaser beam and a laser beam having the minus-first-order laser beam andthe zero-order laser beam in the advancing path and the returning pathare combined and incident on the light receiving device 9. The intervalbetween the signals is determined by the design of the hologram lens 5,not depending on the different protective material thicknesses ofvarious optical discs.

Using such characteristic, the amount of movement d of the objectivelens 6 can be accurately measured, not depending on the change inambient temperature of the optical head 30. The signals E2 and F2obtained by a light whose diffraction order of the hologram lens 5 isdifferent by one order of magnitude only in one of the advancing pathand the returning path. Thus, the d is half of the distance betweenplural focal points which can be actually formed on the optical discside by different orders. When such distance d is divided by the amountof the actuator driving current or voltage changed therebetween, thefocus sensitivity can be determined. To measure the focus sensitivity,the optical disc may be rotated. However, to avoid an error due to thesurface deflection of the optical disc, the optical disc is desirablystopped without being rotated.

After the focus sensitivity is determined in step S3 as described above,in step S4, the objective lens 6 is moved away from the optical disconce. While the optical disc is rotated, the objective lens 6 is broughtclose to the optical disc again. When, in step S5, the signal levelobtained from the light receiving device 9 exceeds the LC, the routineis moved to step S6.

In step S6, the objective lens 6 is brought close to the optical disc bya fixed amount D0. The distance D0 is in the range in which the focuscontrol range is not present. The distance D0 can be determined from theobtained distance d. The protective material thickness of the opticaldisc is T and its refractivity is n. When the wavelength is 405 nm andthe material of the protective material of the optical disc ispolycarbonate, n is about 1.6. The distance from the point at which thesignal G2 is detected is calculated to D0=T/n−(3 d). However, as thedistance D0 is provided so as not to start the focus control by mistaketo the signal H2 appearing following by the signal G2, the distance D0may be from the point at which the signal F2 is detected. In this case,the distance D0 may be D0>2d. As described above, D0 may be in thefollowing range:

2d<D0<T/n−(3d)

When the focus control is performed to the signal H2 by mistake, thefocus control can be stopped once so that the objective lens 6 isbrought close to the optical disc side. Thus, step S6 can be omitted.

Based on the focus sensitivity k determined in step S3, in steps S7 toS10, the upper limit of the amount of movement of the objective lens 6for searching for the signal D1 performing the focus control isdetermined. A distance DS searched for from the position at which thesignal E2 is detected is DS>T/n−(d). Here, T, n, and d are as describedabove. The distance DS from the position at which the signal G2 isdetected is DS>T/n−(3 d) at the minimum. Due to the error factor such asthe surface deflection of the optical disc, the actual distance DS needto be set to be slightly larger than the calculated value. When theinterval between the surface of the objective lens 6 and the opticaldisc surface at read of the optical disc having the protective materialthickness T is the working distance WD and the distance set to beslightly larger is set to be less than the WD, contact of the opticaldisc and the objective lens 6 can be prevented. The distance DS isDS<T/n−(d)+WD at the maximum. Actually, contact of the optical disc andthe objective lens 6 due to the error factor such as the surfacedeflection of the optical disc need to be reliably prevented. So, theamount set to be slightly larger is desirably about one third of the WD.

Desirably,

DS<T/n−(3d)+WD/3

The distance DS of the upper limit of the amount of movement of theobjective lens 6 moved by searching for the signal D1 performing thefocus control is:

T/n−(3d)<DS<T/n−d+WD

In a narrower range,

T/n−d<DS<T/n−(3d)+WD/3

In steps S7 to S10, the objective lens 6 is brought close to the disc(S7) so as to monitor a signal level obtained from the light receivingdevice 9, and when the signal level exceeds the threshold value LE (S8),the focus control is started (S9). When the signal level does not reachthe threshold value LE in step S8, the amount of movement of theobjective lens 6 from the point at which the signal level obtained fromthe light receiving device 9 exceeds the threshold value LC is comparedwith the distance DS (S10). When the amount of movement does not reachthe distance DS, the routine is returned to step S7. When the amount ofmovement reaches the distance DS, the routine is moved to step S11.

What the routine is moved to step S11 means that the signal exceedingthe threshold value LE cannot be detected while the objective lens 6 ismoved in the distance DS. Thus, in step S11, the threshold value LE isset to be lower. For instance, the threshold value LE is set to be abouttwo thirds of the previous one. The routine is moved to step S12. Whenthe routine is moved to step S12, the objective lens 6 has been alreadyclose to the optical disc surface. So, in step S12, while the objectivelens 6 is moved in the reverse direction, that is, in the directionmoving away from the optical disc, the signal level obtained from thelight receiving device 9 is monitored. In step S13, whether or not thesignal level monitored exceeds the threshold value LE is determined andwhen the signal level exceeds the threshold value LE, the focus controlis started (S14). When the signal level does not reach the thresholdvalue LE, the routine is returned to step S5.

As described above, the focus control can be started while contact ofthe objective lens 6 and the optical disc is prevented. In step S6, toimmediately start the focus control, the objective lens 6 is preferablymoved at high speed. On the other hand, in step S7 and thereafter, themoving speed of the objective lens 6 is desirably lowered so as not tomiss the state that the signal level is high.

It is desired that the optical heads 30 and 40 have a mechanism formoving the collimate lens 4 in the optical axis direction in order tochange the amount of spherical aberration of the focusing optical systemaccording to the protective material thickness of the optical disc. Inthis case, in step S2, while the protective material thickness of theoptical disc is determined to be larger than 0.1 mm and the routine ismoved to step S3, that is, between steps S2 and S3, it is desired thatthe collimate lens 4 be moved in such a manner that the amount ofspherical aberration of the plus-first-order diffraction light issuitable to 0.6 mm. By this operation, the amplitude of the signal D1can be maximum so as to start the focus control more stably.

Embodiment 5

FIG. 15 is a schematic diagram of the computer according to anembodiment of the present invention.

In FIG. 15, a computer 500 has the optical disc apparatus 400 accordingto Embodiment 4, an input device 501, such as a keyboard, mouse, ortouch panel, for inputting information, a computation device 502 such asa central processing unit (CPU) for performing computation based oninformation inputted from the input device 501 and information read bythe optical disc apparatus, and an output device 503, such as a CRT,liquid crystal display, or printer, for displaying information such asthe result computed by the computation device 502.

The computer 500 has the optical disc apparatus 400 according toEmbodiment 4, so the computer 500 can satisfactorily perform write orread of plural different kinds of optical discs and can be widelyapplicable.

Embodiment 6

FIG. 16 is a schematic diagram of the optical disc player according toan embodiment of the present invention.

In FIG. 16, an optical disc player 600 has the optical disc apparatus400 according to Embodiment 4, and a conversion device (e.g., a decoder601) from information to an image, for converting an information signalobtained from the optical disc apparatus 400 to an image signal.

The optical disc player 600 can be used as a car navigation system byadding a position sensor such as a GPS and a central processing unit(CPU). The optical disc player 600 can add a display device 602 such asa liquid crystal monitor.

As the optical disc player 600 has the optical disc apparatus 400according to Embodiment 4, the optical disc player 600 cansatisfactorily perform write or read of plural different kinds ofoptical discs and can be widely applicable.

Embodiment 7

FIG. 17 is a schematic diagram of the optical disc recorder according toan embodiment of the present invention.

In FIG. 17, an optical disc recorder 700 has the optical disc apparatus400 according to Embodiment 4, and a conversion device (e.g., an encoder701) from an image to information, for converting image information toan information signal recorded onto the optical disc by the optical discapparatus 400. Desirably, the optical disc recorder 700 also has aconversion device (e.g., a decoder 702) from information to an image,for converting an information signal obtained from the optical discapparatus 400 to image information and can read a recorded image. Theoptical disc recorder 700 may have an output device 703, such as a CRT,a liquid crystal display, and a printer, for displaying information.

As the optical disc recorder 700 has the optical disc apparatus 400according to Embodiment 4, the optical disc recorder 700 cansatisfactorily perform write or read of plural different kinds ofoptical discs and can be widely applicable.

The optical head of the present invention can satisfactorily performwrite or read of plural different kinds of optical discs. The opticaldisc apparatus of the present invention and the computer, optical discplayer, and optical disc recorder having the optical disc apparatus cansatisfactorily perform write or read of plural different kinds ofoptical discs. Thus, they can be widely applicable.

Arbitrary ones of the above various embodiments are combined with eachother, as needed, so as to exhibit the respective effects.

The present invention is sufficiently described in conjunction with thepreferred embodiments with reference to the accompanying drawings. It isapparent that various modifications and corrections can be made forthose skilled in the art. Such modifications and corrections should beunderstood to be included in the scope of the present invention of theattached claims without departing from it.

All of the disclosed contents of the specifications, drawings, claims,and abstracts of Japanese Patent Application No. 2006-331534 filed onDec. 8, 2006 and U.S. Provisional Patent Application No. 60/877,606filed in the United States on Dec. 29, 2006 are incorporated into thisspecification by reference.

1. An optical head comprising: a light source; a diffraction deviceconfigured to diffract a laser beam emitted from the light source so asto generate diffraction lights of plural orders; an objective lens, withn and m as integers, configured to focus an n-order diffraction lightgenerated by the diffraction device onto an information recordingsurface of a first information recording medium, and focus an m-orderdiffraction light onto an information recording surface of a secondinformation recording medium; a condensing lens configured to condensethe laser beam reflected on the information recording surface of thefirst information recording medium or the information recording surfaceof the second information recording medium; and a light receiving unitconfigured to receive the laser beam condensed by the condensing lens soas to generate a focus error signal, wherein the optical path of thelaser beam before being reflected on the information recording medium isan advancing path, the optical path of the laser beam after beingreflected on the information recording medium is a returning path, aposition in the light receiving unit in which a diffraction light withthe n-order diffraction light in the advancing path and the n-orderdiffraction light in the returning path, reflected on the informationrecording surface of the first information recording medium, is focusedby the condensing lens being different from a position in the lightreceiving unit in which a diffraction light different from the n-orderdiffraction light in at least one of the advancing path and thereturning path, reflected on a surface of the first informationrecording medium, is focused by the condensing lens.
 2. The optical headaccording to claim 1, wherein the n and m are different integers, andthe objective lens focuses the laser beams having the same wavelengthemitted from the light source onto the information recording surface ofthe first information recording medium and the information recordingsurface of the second information recording medium.
 3. The optical headaccording to claim 1, wherein the n and mare the same integers, and theobjective lens focuses the laser beams having different wavelengthsemitted from the light source onto the information recording surface ofthe first information recording medium and the information recordingsurface of the second information recording medium.
 4. The optical headaccording to claim 1, wherein the n and m are different integers, andthe objective lens focuses the laser beams having different wavelengthsemitted from the light source onto the information recording surface ofthe first information recording medium and the information recordingsurface of the second information recording medium.
 5. The optical headaccording to claim 1, wherein a position in the light receiving unit inwhich a diffraction light with an m-order diffraction light in theadvancing path and the m-order diffraction light in the returning path,reflected on the information recording surface of the second informationrecording medium, is focused by the condensing lens is different from aposition in the light receiving unit in which a diffraction lightdifferent from the m-order diffraction light in at least one of theadvancing pa h and the returning path, reflected on a surface of thesecond information recording medium, is focused by the condensing lens.6. The optical head according t0 claim 2, wherein a protective materialthickness of the information recording surface of the first informationr cording medium is t1, a protective material thickness of theinformation recording surface of the second information recording mediumis t2, so that t1<t2, and the objective lens focuses a zero-orderdiffraction light onto the information recording surface of the firstinformation recording medium and focuses a plus-first-order diffractionlight onto the information recording surface of the second informationrecording medium.
 7. The optical head according to claim 2, wherein aposition in the light receiving unit in which a diffraction light with azero-order diffraction light in the advancing path and a zero-orderdiffraction light in the returning path, reflected on the informationrecording surface of the first information recording medium, is focusedby the condensing lens is different from a position in the lightreceiving unit in which the zero-order diffraction light in one of theadvancing path and the returning path and a minus-first-orderdiffraction light in the other path, reflected on the surface of thefirst information recording medium, is focused by the condensing lens.8. The optical head according to claim 7, wherein a protective materialthickness of the information recording surface of the first informationrecording medium is t1, a protective material thickness of theinformation recording surface of the second information recording mediumis t2, a refractivity of the protective material is n, a firstprotective material thickness of the information recording surface ofthe first information recording medium is t1_L1, a second protectivematerial thickness of the information recording surface of the firstinformation recording medium is t1_L2, a working distance for write orread of the first information recording medium is WD1, a workingdistance for write or read of the second information recording medium isWD2, and a focal position interval difference between a plus-first-orderdiffraction light and the minus-first-order diffraction light is k, thenWD1−WD2≠(t2−t−2×t1_(—) L1)/n−kandWD1−WD2 ≠(t2−t1−2×t1_(—) L2)/n−k are satisfied.
 9. The optical headaccording to claim 8, whereinWD1−WD2<(t2−t1−2×t1_(—) L1)/n−k is satisfied.
 10. The optical headaccording to claim 2, wherein a position in the light receiving unit inwhich a diffraction light with a plus-first-order diffraction light inthe advancing path and a plus-first-order diffraction light in thereturning path, reflected on the information recording surface of thesecond information recording medium, is focused by the condensing lensis different from a position in the light receiving unit in which aminus-first-order diffraction light in the advancing path and aminus-first-order diffraction light in the returning path, reflected ona surface of the second information recording medium, is focused by thecondensing lens.
 11. The optical head according to claim 10, wherein aprotective material thickness of the information recording surface ofthe first information recording medium is t1, a protective materialthickness of the information recording surface of the second informationrecording medium is t2, a refractivity of the protective material is n,a protective material thickness of the information recording surface ofthe second information recording medium is t2_ra, a working distance forwrite or read of the first information recording medium is WD1, aworking distance for write or read of the second information recordingmedium is WD2, and a focal position interval difference between theplus-first-order diffraction light and the minus-first-order diffractionlight is k, thenWD1−WD2≠(2×t2−2×t1−t2_(—) ra)/(2×n)−k is satisfied.
 12. The optical headaccording to claim 11, whereinWD1−WD2>(2×t2−2×t1−t2_(—) ra)/(2×n)−k is satisfied.
 13. The optical headaccording to claim 2, wherein a protective material thickness of theinformation recording surface of the first information recording mediumis t1, a protective material thickness of the information recordingsurface of the second information recording medium is t2, so that t1<t2,and the objective lens focuses a plus-first-order diffraction light ontothe information recording surface of the first information recordingmedium and focuses a zero-order diffraction light onto the informationrecording surface of the second information recording medium.
 14. Theoptical head according to claim 1, wherein the objective lens is anobjective lens having a diffraction structure integrated with thediffraction device.
 15. The optical head according to claim 14, whereinan effective region of the objective lens has a first region having thediffraction structure and a second region not having the diffractionstructure.
 16. The optical head according to claim 14, wherein aneffective region of the objective lens has a first diffraction region inwhich a predetermined diffraction structure is formed in the innerperiphery region including the optical axis of the objective lens and asecond diffraction region in which a diffraction structure differentfrom that of the first diffraction region is formed around the firstregion.
 17. The optical head according to claim 1, wherein thediffraction device and the objective lens are separated from each other.18. The optical head according to claim 1, wherein a reflectivity of theinformation recording surface of the first information recording mediumis R1 r, and a surface reflectivity of the first information recordingmedium is R1 s, then0.5≦R1r/R1s≦3.0 is satisfied.
 19. The optical head according to claim18, wherein the first information recording medium has a plurality ofinformation recording surfaces.
 20. An objective lens having adiffraction structure configured to diffract a laser beam emitted from alight source so as to generate diffraction lights of a plurality oforders, with n and m as integers, focus an n-order diffraction lightgenerated by the diffraction structure onto an information recordingsurface of a first information recording medium, and focus an m-orderdiffraction light onto an information recording surface of a secondinformation recording medium, wherein an optical path of the laser beambefore being reflected on the information recording medium is anadvancing path, an optical path of the laser beam after being reflectedon the information recording medium is a returning path, a position inwhich a diffraction light with the n-order diffraction light in theadvancing path and the n-order diffraction light in the returning path,reflected on the information recording surface of the first informationrecording medium, is focused by a condensing lens for focusing the laserbeam reflected on the information recording surface of the firstinformation recording medium or the information recording surface of thesecond information recording medium is different from a position inwhich a diffraction light different from the n-order diffraction lightin at least one of the advancing path and the returning path, reflectedon the surface of the first information recording medium, is focused bythe condensing lens.
 21. A diffraction device having a diffractionstructure configured to diffract a laser beam emitted from a lightsource so as to generate diffraction lights of plural orders, in whichwith n and m as integers, an n-order diffraction light generated by thediffraction structure being focused onto an information recordingsurface of a first information recording medium and an m-orderdiffraction light being focused onto an information recording surface ofa second information recording medium by an objective lens, wherein anoptical path of the laser beam before being reflected on the informationrecording medium is an advancing path, an optical path of the laser beamafter being reflected on the information recording medium is a returningpath, a position in which a diffraction light with the n-orderdiffraction light in the advancing path and the n-order diffractionlight in the returning path, reflected on the information recordingsurface of the first information recording medium, is focused by acondensing lens for focusing the laser beam reflected on the informationrecording surface of the first information recording medium or theinformation recording surface of the second information recording mediumbeing different from a position in which a diffraction light differentfrom the n-order diffraction light in at least one of the advancing pathand the returning path, reflected on a surface of the first informationrecording medium, is focused by the condensing lens.
 22. An optical discapparatus comprising an optical head, a motor for rotating and drivingan information recording medium, and a control unit for controlling theoptical head and the motor, the optical head comprising: a light source;a diffraction device configured to diffract a laser beam emitted fromthe light source so as to generate diffraction lights of plural orders;an objective lens, with n and m as integers, configured to focus ann-order diffraction light generated by the diffraction device onto aninformation recording surface of a first information recording medium,and focus an m-order diffraction light onto an information recordingsurface of a second information recording medium; a condensing lensconfigured to condense the laser beam reflected on the informationrecording surface of the first information recording medium or theinformation recording surface of the second information recordingmedium; and a light receiving unit configured to receive the laser beamcondensed by the condensing lens so as to generate a focus error signal,wherein the optical path of the laser beam before being reflected on theinformation recording medium is an advancing path, the optical path ofthe laser beam after being reflected on the information recording mediumis a returning path, a position in the light receiving unit in which adiffraction light with the n-order diffraction light in the advancingpath and the n-order diffraction light in the returning path, reflectedon the information recording surface of the first information recordingmedium, is focused by the condensing lens being different from aposition in the light receiving unit in which a diffraction lightdifferent from the n-order diffraction light in at least one of theadvancing path and the returning path, reflected on a surface of thefirst information recording medium, is focused by the condensing lens.23. An optical disc apparatus comprising: an optical head having a lightsource, an objective lens configured to focus a laser beam emitted fromthe light source onto an information recording surface of an informationrecording medium, a light receiving unit configured to receive the laserbeam reflected on the information recording surface so as to generate afocus error signal, and an actuator for moving the objective lens in afocus direction; and a control unit configured to control the opticalhead, the control unit determining an amount of movement of theobjective lens based on a focus sensitivity in the relation between anelectric current or a voltage to be supplied to the actuator and anamount of movement of the objective lens and starting focus control whena focus error signal obtained with movement of the objective lens in thedirection close to the information recording medium exceeds a thresholdvalue.
 24. The optical disc apparatus according to claim 23, furthercomprising a memory configured to store the focus sensitivity, whereinthe control unit reads the focus sensitivity from the memory.
 25. Theoptical disc apparatus according to claim 23, wherein the control unitmeasures the focus sensitivity using the focus error signal.